v3(services) Added request tests for async app to service binding (cloudfoundry#1889)

* v3(services) Added request tests for async app to service binding

Create service credential binding for type app is now tested in request spec.

[174119280](https://www.pivotaltracker.com/story/show/174119280)
[174120249](https://www.pivotaltracker.com/story/show/174120249)

* Fix formatting issues

Author: FelisiaM

app/controllers/v3/service_credential_bindings_controller.rb

@@ -62,8 +62,6 @@ def create
     end
   rescue V3::ServiceCredentialBindingCreate::UnprocessableCreate => e
     unprocessable!(e.message)
-  rescue V3::ServiceCredentialBindingCreate::Unimplemented
-    head :not_implemented
   end
 
   def destroy