make handleAs ignore response body for status code 204

[tostrowski]

When response is 204 and handleAs="json" then core-ajax still tries to parse response body which results in SyntaxError: Unexpected end of input {stack: (...), message: "Unexpected end of input"}
Maybe it could set response field to undefined instead.

[DavidLGoldberg]

Should fire an error no?

More opinions:
http://stackoverflow.com/questions/11970962/valid-json-in-response