allow to call enhancer previews thanks to an hmvc call

Evenn if it seemed odd at first, I decided to use a "not" in "if not hmvc" so that the method ends with the "return" statement.

Note 1 : To make this efficient, action_save() must return content when it's overridden.

Note 2 : was'nt sure of doing it for Elche or other version; made it only for the latest as it's a new feature and not a bugfix