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
Merge pull request #12112 from adamniedzielski/fix-form-for-namespace-and-as

form_for - fix :namespace and :as options clash

Conflicts:
	actionview/CHANGELOG.md