Inexplicable KeyError in pd.io.json.json_normalize(dic)
#### Code Sample, a copy-pastable example if possible

```python

import pandas as pd

dic = {'_id': '5436e3abbae478396759f0cf', 'meta': {'clinical': {'benign_malignant': 'benign', 'age_approx': 55, 'sex': 'female', 'diagnosis': 'nevus', 'diagnosis_confirm_type': None, 'anatom_site_general': 'anterior torso', 'melanocytic': True}, 'acquisition': {'image_type': 'dermoscopic', 'pixelsX': 1022, 'pixelsY': 767}}, 'name': 'ISIC_0000000'}

frame = pd.io.json.json_normalize(dic)

```

#### Problem description

version 0.23.0 throws an

`KeyError: 'diagnosis_confirm_type'`

which 0.22.0 did not. 

#### Expected Output

No exception

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
