Implement autocorrection for Rails/ReadWriteAttribute cop.

This implements autocorrection for the Rails/ReadWriteAttribute cop.

It is my first contribution to rubocop, therefore I'm not sure it fits your coding conventions;-)
Move these 3 duplicated lines into the `autocorrect` method and return the "selfing" value. After that change it would be better to name the methods, for example, `..._attribute_replacement`.