Csv download improvements

[andrewhn]

Hi,

This PR does two things:

• names csv and json files (with extension) via a Content-Disposition header
• includes a header in CSV output where data is meaningful (i.e. index is not generic). Currently, for example, time series data is outputted without timestamps and pivot table data is exported without row names

[landscape-bot]

Code quality remained the same when pulling 1c840f7 on andrewhn:csv_download_improvements into 81de51b on airbnb:master.

[coveralls]

Coverage increased (+0.03%) to 71.338% when pulling 1c840f7 on andrewhn:csv_download_improvements into 81de51b on airbnb:master.

[landscape-bot]

Code quality remained the same when pulling f6afaa3 on andrewhn:csv_download_improvements into 3175882 on airbnb:master.

[coveralls]

Coverage increased (+0.03%) to 71.302% when pulling f6afaa3 on andrewhn:csv_download_improvements into 3175882 on airbnb:master.

[landscape-bot]

Code quality remained the same when pulling d049199 on andrewhn:csv_download_improvements into c2baa53 on airbnb:master.

[coveralls]

Coverage increased (+0.03%) to 71.302% when pulling d049199 on andrewhn:csv_download_improvements into c2baa53 on airbnb:master.

[mistercrunch]

LGTM, thanks for the PR