py3: ensure the HTTP password manager returns strings, not bytes The digest handler calls into the password manager on its own, and it apparently expects strings. Perhaps the Basic authentication handler didn't hit this because of its manual password fetch and format in retry_http_basic_auth(). The `pycompat.bytesurl()` on the user and password just above the first url.py diff seems unnecessary, because the password proxy in ui is converting to bytes IIUC.

tests: enable HTTP digest testing I suppose we could spin the client side extension off to a *.py file if it gets more use. I was basically just looking to avoid killing the server and relaunching it just to change authentication schemes, because that doesn't always work on Windows. The test changes capture the problem with py3.

wsgiheaders: make sure __repr__() returns a string When printing `req.headers` on the server side to debug, it complained that '%b' needed to take a string, not bytes. Changing '%s' to '%r' caused it to complain that __repr__ didn't return a string.