tomchristie: Running the example in #652 after this change...

```python
$ python ./example.py 
requests {'a': '1', 'b': '2'}
httpx {'a': '1', 'b': '2'}
```
diff --git a/httpx/models.py b/httpx/models.py
index ad3599b..fc938c3 100644
--- a/httpx/models.py
+++ b/httpx/models.py
@@ -90,9 +90,14 @@ class URL:
         if self.is_absolute_url:
             self._uri_reference = self._uri_reference.normalize()
 
-        # Add any query parameters.
+        # Add any query parameters, merging with any in the URL if needed.
         if params:
-            query_string = str(QueryParams(params))
+            if self._uri_reference.query:
+                url_params = QueryParams(self._uri_reference.query)
+                url_params.update(params)
+                query_string = str(url_params)
+            else:
+                query_string = str(QueryParams(params))
             self._uri_reference = self._uri_reference.copy_with(query=query_string)
 
         # Enforce absolute URLs by default.

`params` overrides query string in url
Hi,

I noticed a difference between `httpx` and `requests` when processing query parameters. I provided the `params` for the url with query string, `requests` would **merge** `params` into query string, but `httpx` replaces the whole query string.

Here is a simple script:

```py
import requests
import httpx
import asyncio

URL = 'https://httpbin.org/get?a=1'

r = requests.get(URL, params={'b': 2})
print('requests', r.json()['args'])

async def f():
    r = await httpx.get(URL, params={'b': 2})
    print('httpx', r.json()['args'])
asyncio.run(f())
```

Results:
```
requests {'a': '1', 'b': '2'}
httpx {'b': '2'}
```

Is this an intended behavior?
diff --git a/tests/models/test_url.py b/tests/models/test_url.py
index 1415ee9..491d0a4 100644
--- a/tests/models/test_url.py
+++ b/tests/models/test_url.py
@@ -89,7 +89,7 @@ def test_url_params():
     assert str(url) == "https://example.org:123/path/to/somewhere?a=123"
 
     url = URL("https://example.org:123/path/to/somewhere?b=456", params={"a": "123"})
-    assert str(url) == "https://example.org:123/path/to/somewhere?a=123"
+    assert str(url) == "https://example.org:123/path/to/somewhere?b=456&a=123"
 
 
 def test_url_join():

pip install -e .[dev]
parse_log_pytest
requirements.txt
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
aspy.refactor-imports==3.0.2
attrs==25.3.0
autoflake==2.3.1
babel==2.17.0
backrefs==5.8
black==25.1.0
brotlipy==0.7.0
certifi==2025.1.31
cffi==1.17.1
chardet==3.0.4
charset-normalizer==3.4.1
click==8.1.8
colorama==0.4.6
coverage==7.8.0
cryptography==44.0.2
exceptiongroup==1.2.2
flake8==7.2.0
flake8-bugbear==24.12.12
flake8-pie==0.16.0
ghp-import==2.1.0
h11==0.8.1
h2==3.2.0
hpack==3.0.0
hstspreload==2025.1.1
-e git+https://github.com/encode/httpx.git@5ee512d803d1d6b49dc171e1114f0075618de78e#egg=httpx
hyperframe==5.2.0
idna==2.10
importlib_metadata==8.6.1
iniconfig==2.1.0
isort==6.0.1
Jinja2==3.1.6
Markdown==3.7
MarkupSafe==3.0.2
mccabe==0.7.0
mergedeep==1.3.4
mkautodoc==0.2.0
mkdocs==1.6.1
mkdocs-get-deps==0.2.0
mkdocs-material==9.6.10
mkdocs-material-extensions==1.3.1
mypy==1.15.0
mypy-extensions==1.0.0
outcome==1.3.0.post0
packaging==24.2
paginate==0.5.7
pathspec==0.12.1
platformdirs==4.3.7
pluggy==1.5.0
pycodestyle==2.13.0
pycparser==2.22
pyflakes==3.3.1
Pygments==2.19.1
pymdown-extensions==10.14.3
pytest==8.3.5
pytest-asyncio==0.26.0
pytest-cov==6.0.0
pytest-trio==0.8.0
python-dateutil==2.9.0.post0
PyYAML==6.0.2
pyyaml_env_tag==0.1
requests==2.32.3
rfc3986==1.5.0
seed-isort-config==2.2.0
six==1.17.0
sniffio==1.3.1
sortedcontainers==2.4.0
tomli==2.2.1
trio==0.29.0
trustme==1.2.1
typing_extensions==4.13.0
urllib3==2.3.0
uvicorn==0.34.0
watchdog==6.0.0
zipp==3.21.0

name: httpx
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - aspy-refactor-imports==3.0.2
      - attrs==25.3.0
      - autoflake==2.3.1
      - babel==2.17.0
      - backrefs==5.8
      - black==25.1.0
      - brotlipy==0.7.0
      - certifi==2025.1.31
      - cffi==1.17.1
      - chardet==3.0.4
      - charset-normalizer==3.4.1
      - click==8.1.8
      - colorama==0.4.6
      - coverage==7.8.0
      - cryptography==44.0.2
      - exceptiongroup==1.2.2
      - flake8==7.2.0
      - flake8-bugbear==24.12.12
      - flake8-pie==0.16.0
      - ghp-import==2.1.0
      - h11==0.8.1
      - h2==3.2.0
      - hpack==3.0.0
      - hstspreload==2025.1.1
      - hyperframe==5.2.0
      - idna==2.10
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - isort==6.0.1
      - jinja2==3.1.6
      - markdown==3.7
      - markupsafe==3.0.2
      - mccabe==0.7.0
      - mergedeep==1.3.4
      - mkautodoc==0.2.0
      - mkdocs==1.6.1
      - mkdocs-get-deps==0.2.0
      - mkdocs-material==9.6.10
      - mkdocs-material-extensions==1.3.1
      - mypy==1.15.0
      - mypy-extensions==1.0.0
      - outcome==1.3.0.post0
      - packaging==24.2
      - paginate==0.5.7
      - pathspec==0.12.1
      - platformdirs==4.3.7
      - pluggy==1.5.0
      - pycodestyle==2.13.0
      - pycparser==2.22
      - pyflakes==3.3.1
      - pygments==2.19.1
      - pymdown-extensions==10.14.3
      - pytest==8.3.5
      - pytest-asyncio==0.26.0
      - pytest-cov==6.0.0
      - pytest-trio==0.8.0
      - python-dateutil==2.9.0.post0
      - pyyaml==6.0.2
      - pyyaml-env-tag==0.1
      - requests==2.32.3
      - rfc3986==1.5.0
      - seed-isort-config==2.2.0
      - six==1.17.0
      - sniffio==1.3.1
      - sortedcontainers==2.4.0
      - tomli==2.2.1
      - trio==0.29.0
      - trustme==1.2.1
      - typing-extensions==4.13.0
      - urllib3==2.3.0
      - uvicorn==0.34.0
      - watchdog==6.0.0
      - zipp==3.21.0
prefix: /opt/conda/envs/httpx

tests/models/test_url.py::test_url_params
tests/models/test_url.py::test_idna_url[http_with_port] tests/models/test_url.py::test_idna_url[unicode_tr46_compat] tests/models/test_url.py::test_idna_url[https_without_port] tests/models/test_url.py::test_idna_url[https_with_port] tests/models/test_url.py::test_idna_url[http_with_custom_port] tests/models/test_url.py::test_idna_url[https_with_custom_port] tests/models/test_url.py::test_url tests/models/test_url.py::test_url_eq_str tests/models/test_url.py::test_url_join tests/models/test_url.py::test_url_join_rfc3986 tests/models/test_url.py::test_url_set tests/models/test_url.py::test_url_full_path_setter tests/models/test_url.py::test_origin_from_url_string tests/models/test_url.py::test_url_copywith_for_authority
httpx/models.py
httpx/models.py