Added an `enforce_utf8` hash option for `form_tag` method

Control to output a hidden input tag with name `utf8` without monkey
patching

Before:

    form_tag
    # => '<form>..<input name="utf8" type="hidden" value="&#x2713;" />..</form>'

After:

    form_tag
    # => '<form>..<input name="utf8" type="hidden" value="&#x2713;" />..</form>'

    form_tag({}, { :enforce_utf8 => false })
    # => '<form>....</form>'