Fix typo in set_callback docs. [ci skip]
Merge pull request #12148 from gzohari/callback-typo

Fix typo in set_callback docs. [ci skip]
Do not use instance variables if they are not reused elsewhere
form_for - fix :namespace and :as options clash

:as option should not overwrite :namespace option when
generating html id attribute of the form element. id should be prefixed
by specified namespace even if :as option is present

Add test case showing the issue and code fixing it