Transform ajax response to json only if no responseType specified or if this responseType is specifically json

I realize that even if I send a request with a responseType="text", angular would transform response into an object if it looks like json.

I think angular should do that only the responseType is absent or if it is === "json"

That's what this pull request is doing
Having another way to specify this from the client-side would be good too, to deal with bad backends, but we can't use `XMLHttpRequest#responseType` for this, because of the browser issue mentioned above