renderJson use text/plain if application/json is not available

We use jQueryFileUpload in a project where we faced a problem in connection with headers.
Basicly this plugin in IE and Opera uses iframe solution for file uploads. The problem is that these browsers don't support <code>application/json</code> and require <code>text/json</code> for response.

https://github.com/blueimp/jQuery-File-Upload/wiki/Setup#content-type-negotiation

The solution is simply just checking whether we can send <code>application/json</code> header or not.