If a browser encounters a select element without any selected option element, it automatically pre-selects the first one. If multiple options are selected, all but the last one get deselected.

This is a fix for bug #1665241, as the original fix for #399249
in 0b14af50fdc878199f8a3be3053eef42d3e9851f should not apply to
option values, which are supposed to be passed through as-is.

In Python 3, urlencode() returns Unicode string; but urlopen() wants
only bytes in data, so it need to be encoded first.

Fixes:

    >>> form = lxml.html.fromstring('<form method="POST">', base_url='http://localhost')
    >>> lxml.html.submit_form(form)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File ".../lxml/html/__init__.py", line 1119, in submit_form
        return open_http(form.method, url, values)
      File ".../lxml/html/__init__.py", line 1140, in open_http_urllib
        return urlopen(url, data)
      File ".../urllib/request.py", line 163, in urlopen
        return opener.open(url, data, timeout)
      File ".../urllib/request.py", line 464, in open
        req = meth(req)
      File ".../urllib/request.py", line 1183, in do_request_
        raise TypeError(msg)
    TypeError: POST data should be bytes or an iterable of bytes. It cannot be of type str.

Ignore disabled form inputs

Similar to `submit`, `image` and `reset`, browsers don't send `file` field values in the POST when form is submitted. `FormElement.form_values` method already correctly excluded `submit`, `image` and `reset` fields, now it also excludes the `file` fields.