Add automated tests for JSONP responses

[strk]

There seem to be no acceptance tests for JSONP requests. The tests should check all formats for which providing a callback is supported and they should test the HTTP response codes for non-success results to have, in the callback case, still a 200 response code.

Lack of these tests allowed 1.10.0 to be released with a regression about callbacks.

\cc @rochoa

[strk]

NOTE: the regressing feature has been moved into the CDB-2600 branch ( https://github.com/CartoDB/CartoDB-SQL-API/tree/CDB-2600 ) -- the branch will need to be merged the new tests as soon as available, and ensured to work with them before the feature is merged back to master