Pass enforce_utf8 option from form_for to form_tag

Currently `form_for` invokes `form_tag` passing all the options accepted
by `form_tag` except for one: `enforce_utf8`.

This option was added in 06388b0 to allow users to prevent the hidden
field with the UTF-8 character from appearing in the output.

This commit implements the same behavior also for `form_for`.