Add csv response content type to http query endpoint

[corylanou]

Adds ability to get http query responses back in csv format.

Actual data:

Using database foo
> select * from cpu
name: cpu
---------
time                    rack    region  val
1469735393542775676                     1
1469740758548191803     200     east    1

Curl command issued:

$ curl -H "Content-Type: application/csv" -G http://localhost:8086/query --data-urlencode "db=foo" --data-urlencode "pretty=true" --data-urlencode "q=SELECT * FROM foo..cpu"

Response:

name,time,rack,region,val
cpu,2016-07-28 19:49:53.542775676 +0000 UTC,,,1
cpu,2016-07-28 21:19:18.548191803 +0000 UTC,200,east,1

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

/cc @jsternberg

[mention-bot]

@corylanou, thanks for your PR! By analyzing the annotation information on this pull request, we identified @e-dard, @gunnaraasen and @joelegasse to be potential reviewers

[jsternberg]

This looks similar to #6815 and that one should probably be merged first so this one can be based off of that.

[corylanou]

Per our conversation, it's based on the branch you are referencing. If you have it already done, we can delete this PR.

[corylanou]

closing in favor of #7099