v3(services): duplicate route service binding should return 422

[#174085457](https://www.pivotaltracker.com/story/show/174085457)

Author: Brian Butz

app/controllers/v3/service_route_bindings_controller.rb

@@ -83,6 +83,6 @@ def route_services_disabled!
   end
 
   def already_exists!
-    raise CloudController::Errors::ApiError.new_from_details('ServiceInstanceAlreadyBoundToSameRoute')
+    raise CloudController::Errors::ApiError.new_from_details('ServiceInstanceAlreadyBoundToSameRoute').with_response_code(422)
   end
 end

spec/request/service_route_bindings_spec.rb

@@ -196,7 +196,7 @@
 
         it 'fails with a specific error' do
           api_call.call(space_dev_headers)
-          expect(last_response).to have_status_code(400)
+          expect(last_response).to have_status_code(422)
 
           expect(parsed_response['errors']).to include(
             include({