Fix predicate method name for enum attributes by appending name of the enum attribute to the generated method

This tries to fix #17415. I just added code and fixed existing tests. Please let me know your thoughts on this alternative. 

Thanks to @guilhermeFranco for suggesting this here - https://github.com/rails/rails/issues/17415#issuecomment-61639801. I will add you to the changelog if this is acceptable solution.