Unescaped quote marks in to_json output when run on multi indexed object
#### Code Sample

```python
import pandas as pd
import numpy as np

arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'], ['one',
    'two', 'one', 'two', 'one', 'two', 'one', 'two']]

tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
series = pd.Series(np.random.randn(8), index=index)

print(series.to_json())
```
Output:
```json
{"["bar","one"]":-0.0591166327,"["bar","two"]":0.871145093,"["baz","one"]":0.61280938,"["baz","two"]":0.3848564991,"["foo","one"]":-0.8986592304,"["foo","two"]":-0.4631529084,"["qux","one"]":-1.3482521044,"["qux","two"]":0.5209236198}
```
#### Problem description

The output contains unescaped quotation marks, which makes the output invalid json.

#### Expected Output
Quote marks should be escaped, for example:
```json
{"[\"bar\",\"one\"]":-0.0591166327,"[\"bar\",\"two\"]":0.871145093,"[\"baz\",\"one\"]":0.61280938,"[\"baz\",\"two\"]":0.3848564991,"[\"foo\",\"one\"]":-0.8986592304,"[\"foo\",\"two\"]":-0.4631529084,"[\"qux\",\"one\"]":-1.3482521044,"[\"qux\",\"two\"]":0.5209236198}
```

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

I reproduced it in pandas install from master:
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.8-200.fc24.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8

pandas: 0.19.0+401.g8452080
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.0
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_datareader: None

</details>
