Add responseJSON attribute to JQueryXHR
Update jquery-tests.ts

Fix spelling...
Update jquery.d.ts

Make responseJSON optional, since it is only present on jqXHR if the Content-Type of the response is "application/json".
Since responseJSON is an "output" parameter on jqXHR, I don't think it makes any functional difference, if it is marked as optional or not, but logically it probably makes more sense...
Merge pull request #2324 from noxhoej/master

Add responseJSON attribute to JQueryXHR