add json patch update method and specs for users_controller update

I think this is a good start for working with JSON patch updates although I am not 100% sure about failed responses. I've tried to conform to the HTTP specs and provide error messages in the response body.
So JSON API doesn't require you to support every PATCH operation http://jsonapi.org/format/#updating-a-document. 

I think it also requires content headers like:
```
Accept: application/vnd.api+json; verison=1
Content-Type: applicaiton/json-patch+json
```
I also think the gem you're using will let you do the patch operations using ruby hashes instead of JSON string, which seems like it would be faster https://github.com/guillec/json-patch#usage