pd.DataFrame.replace regression causes dtype to remain object
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

# broken after pandas 0.23.4 if only "a" is replaced
def return_replace(initial):
	return initial.replace({"a": 1.0, "b": 0.0})

# working
def return_replace_just_one(initial):
	return initial.replace({"a": 1.0})

# the following should all be float64
print("problem:", return_replace(pd.DataFrame(["a"])).dtypes[0])
print("works:", return_replace(pd.DataFrame(["b"])).dtypes[0])
print("works:", return_replace_just_one(pd.DataFrame(["a"])).dtypes[0])
print("works:", return_replace(pd.DataFrame(["a", "b"])).dtypes[0])
```
#### Problem description

The behaviour shown above is inconsistent and hard to spot. In my case, it broke one of my tests due to mismatching types (as I was expecting a float64).
The problem seems to involve a regression when upgrading from pandas 0.23.4 to any later version (tested with 0.24.0, 0.24.1 and 0.24.2, all of which have the same issue).
Returning back to the old behaviour of changing the type also in the first case where it fails above (i.e. changing the type whenever possible?) would be more consistent and not require a manual type definition.

This is probably related to https://github.com/pandas-dev/pandas/issues/23305.

#### Expected Output

```
problem: float64
works: float64
works: float64
works: float64
```

#### Output of ``pd.show_versions()``

Note: The below is for the working version of pandas!

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 5.1.6-arch1-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8
LOCALE: de_DE.UTF-8

pandas: 0.23.4
pytest: 4.4.2
pip: 19.1.1
setuptools: 41.0.1
Cython: None
numpy: 1.16.4
scipy: 1.3.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
