Have pick_avatar return json.

I'm working on writing a test in the discourse_api gem for uploading
avatars and the pick method needs to return a json response.

I also added a test to make sure json is returned.

spec/controllers/users_controller_spec.rb

@@ -1292,6 +1292,13 @@ describe UsersController do
         response.should be_success
       end
 
+      it 'returns success' do
+        xhr :put, :pick_avatar, username: user.username, upload_id: 111
+        user.reload.uploaded_avatar_id.should == 111
+        response.should be_success
+        json = ::JSON.parse(response.body)
+        json['success'].should == "OK"
+      end
     end
 
   end