JSONResponseWriter should return Content-Type application/javascript for JSONP requests

The JSONResponseWriter handles two purposes:

• writing JSON responses (wt=json)
• writing JSONP responses, namely JSON responses wrapped by a JavaScript function (wt=json&json.wrf=myFunction)

The response writer returns the same Content-Type in both cases. (default: application/json)
But for JSONP the correct Content-Type would be "application/javascript".

The response type is configurable, but it is currently not possible to return the correct Content-Type in both cases with one configuration at the same time.

The attached patch changes the default Content-Type to application/javascript; charset=utf-8 for responses with wrapper functions (JSONP). If param content-type is configured, this Content-Type will be returned for both JSON and JSONP to ensure backward compatibility.