PY3 port headers_raw_to_dict and headers_dict_to_raw to Python 3

It turns out w3lib.http was not ported. Tests passed because there was not enough tests.

The change could be backwards incompatible because these functions now require bytestrings and return bytestrings. But Scrapy always pass bytestrings to them, and in Python 2.x 'strings' are already bytestrings, so it shouldn't break much code.