added unprocessableEntity() to ResponseEntity

https://jira.spring.io/browse/SPR-12515

Lately I'm using http 422 Unprocessable Entity for returning validation errors. instead of a 400. It seems many public api's are changing to this (like github api).
So please add a static unprocessableEntity() builder method to ResponseEntity to have a shortcut for returning this http code.
This just like earlier request like SPR-12070 and SPR-12112.