hgweb: parse and store HTTP request headers
WSGI transmits HTTP request headers as HTTP_* environment variables.
We teach our parser about these and hook up a dict-like data
structure that supports case insensitive header manipulation.
Differential Revision: https://phab.mercurial-scm.org/D2742
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program