Series.to_json - invalid handling of quotes
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

pd.DataFrame([
  {'A':'a', 'B':'b'}
]).groupby(
  ['A','B']
).apply(
  lambda x: x.to_dict(orient='records')
).to_json()
```
produces an invalid JSON string:

```python
'{"["a","b"]":[{"A":"a","B":"b"}]}'
```

#### Problem description

For the given transformation, the output of `to_json` of a `pandas.core.series.Series` is not a valid JSON object.

Would be nice if it was.

#### Expected Output

```python
'{"[\'a\',\'b\']":[{"A":"a","B":"b"}]}'
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.84
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.2
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.0
pip              : None
setuptools       : None
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : None
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.10
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : None
</details>
