wireprotoserver: headers are bytes for us internally, use bytes
This re-fixes test-pull-http.t on Python 3. Probably many others as well.
Differential Revision: https://phab.mercurial-scm.org/D3249
hgweb: put response headers back into str for Python 3
This fixes a lot of hanging tests on Python 3, because
"Content-Length" was getting sent as "b'Content-Length'" (yes, really)
and then clients would expect a close-is-end body instead of counting
off a certain number of bytes.
Differential Revision: https://phab.mercurial-scm.org/D3248