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
this is not the right way to make this change. On some browsers, we actually __cant__ set `responseType` to `json` even if we try (see: safari 6/older versions of safari 7 before this was fixed in webkit). I have a CL to only use the default JSON deserialization if we have a string and content-type is json.

Feel free to take a look at #5765 for this. It's a bit bitrotten now, but web services are far more likely to specify the content-type header (most will do it automatically) than users are to explicitly set `responseType=json` in their $http config