Begin to fix type confusion in HTTPHeaders.

Begin to fix type confusion in HTTPHeaders.

Values in HTTPHeaders were typed inconsistently: the class itself
assumed that values were of type str, while many callers assumed they
were bytes. Since headers are practically restricted to ascii, this
would work on python 2 but fail when certain combinations were
encountered on python 3 (notably the combination of GzipContentEncoding
with multiple preexisting Vary headers).

This commit adds a test and fixes the bug, and also adds enough type
annotations that mypy was able to report errors in the old incorrect
code.

Fixes #1670