Passing a unicode url to get() always worked (assuming the url contained only ASCII), but it didn't work to post(), put(), or delete() if query parameters were included. This change fixes the latter cases.

As the test shows, this fixes passing boths params and upload_files on Python 3.

It would fail if the length of bytes and unicode differs.

It now passes non-bytes to `Form` directly.

* py3 fixes for form handling in paste.fixture

It uses "not six.PY2" in contrast to other places in the code to be
forward-compatible.

I've not looked too closely, but it might make sense to decode body/text
in the beginning already, instead of having it as bytes internally?!

Also, like mentioned in [1] already, it should probably use the correct
source encoding?!

1: https://github.com/cdent/paste/blob/36e5b8bd16a6063ec654faf04541f3a20d19f7fe/paste/fixture.py#L820

* Add test, using/fixing SlowConsumer form app

See https://www.python.org/dev/peps/pep-0479/