json_normalize gives KeyError in 0.23
#### Code Sample, a copy-pastable example if possible

```python
import json
from pandas import show_versions
from pandas.io.json import json_normalize

print(show_versions())

with open('test.json', 'r') as infile:
    d = json.load(infile)

normed = json_normalize(d)
```

The `test.json` file is rather lengthy, with a structure similar to:

```json
{
  "subject": {
    "pairs": {
      "A1-A2": {
        "atlases": {
          "avg.corrected": {
            "region": null,
            "x": 49.151580810546875,
            "y": -33.148521423339844,
            "z": 27.572303771972656
          }
        }
      }
    }
  }
}
```

This minimal version is enough to show the error below.

#### Problem description

This problem is *new* in pandas 0.23. I get the following traceback:

Traceback:

```pytb
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    normed = json_normalize(d)
  File "/Users/depalati/miniconda3/lib/python3.6/site-packages/pandas/io/json/normalize.py", line 203, in json_normalize
    data = nested_to_record(data, sep=sep)
  File "/Users/depalati/miniconda3/lib/python3.6/site-packages/pandas/io/json/normalize.py", line 88, in nested_to_record
    new_d.update(nested_to_record(v, newkey, sep, level + 1))
  File "/Users/depalati/miniconda3/lib/python3.6/site-packages/pandas/io/json/normalize.py", line 88, in nested_to_record
    new_d.update(nested_to_record(v, newkey, sep, level + 1))
  File "/Users/depalati/miniconda3/lib/python3.6/site-packages/pandas/io/json/normalize.py", line 88, in nested_to_record
    new_d.update(nested_to_record(v, newkey, sep, level + 1))
  File "/Users/depalati/miniconda3/lib/python3.6/site-packages/pandas/io/json/normalize.py", line 84, in nested_to_record
    new_d.pop(k)
KeyError: 'region'
```

Note that running the same code on pandas 0.22 does not result in any errors. I suspect this could be related to #20399.

#### Expected Output

Expected output is a flattened `DataFrame` without any errors.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0
pytest: 3.5.1
pip: 9.0.1
setuptools: 38.4.0
Cython: None
numpy: 1.14.2
scipy: None
pyarrow: None
xarray: 0.10.3
IPython: 6.3.1
sphinx: 1.7.2
patsy: None
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
