DataFrame.to_json silently ignores index parameter for most orients.
*see 
https://github.com/pandas-dev/pandas/issues/25513#issuecomment-469264925*

#### Code Sample, a copy-pastable example if possible

```python
from __future__ import print_function

import pandas as pd

df = pd.DataFrame.from_records(
    [
        {'id': 'abc', 'data': 'qwerty'},
        {'id': 'def', 'data': 'uiop'}
    ],
    index='id'
)

print(df.to_json(orient='records', index=True))

# Prints out:
[{"data":"qwerty"},{"data":"uiop"}]

series = df.squeeze()
print(series.to_json(orient='records', index=True))

# Prints out:
["qwerty","uiop"]
```
#### Problem description

When creating a `DataFrame` that has two columns, one to be used as an index and another for the data, if you call `.to_json(orient='records')` the index is omitted. I know that in theory I should be using a `Series` for this, but I'm using it to convert a CSV file into JSONL and I don't know what the CSV file is going to look like ahead of time.

In any case, squeezing the `DataFrame` into a `Series` doesn't work either. In fact, the bug in `Series.to_json` is even worse, as it produces an array of strings instead of an array of dictionaries.

This bug is present in master.

#### Expected Output

Expected output is:

```json
[{"id":"abc", "data":"qwerty"},{"id":"def","data":"uiop"}]
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.1
pytest: 4.3.0
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.6
numpy: 1.16.2
scipy: None
pyarrow: 0.12.1
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: 2.7.7 (dt dec pq3 ext lo64)
jinja2: None
s3fs: 0.2.0
fastparquet: 0.2.1
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
