Add `:enforce_utf8` option to form_for

Since 06388b0 `form_tag` accepts the option `enforce_utf8` which, when set to
false, prevents the hidden "UTF8 enforcer" field from appearing in the output.

This commit implements the same behavior for `form_for`.

Stems from https://github.com/rails/rails/pull/17685#issuecomment-63871395