Repeal `autocomplete` form attribute being boolean

`autocomplete` attribute may have string values:
http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#attr-fe-autocomplete
#6464 - assertions should fail if attribute is being prefixed by `data-`
#6464 - autocomplete is not a boolean value anymore
#6464 - autocomplete supports non-boolean values (behaves like a text attribute)
Merge branch 'hotfix/#6464-allow-non-boolean-autocomplete-attribute-values' into develop

Close #6464