Uwsgi: combining headers with identical names (ticket #1724).

Uwsgi: combining headers with identical names (ticket #1724).

The uwsgi specification states that "The uwsgi block vars represent a
dictionary/hash".  This implies that no duplicate headers are expected.

Further, provided headers are expected to follow CGI specification,
which also requires to combine headers (RFC 3875, section "4.1.18.
Protocol-Specific Meta-Variables"): "If multiple header fields with
the same field-name are received then the server MUST rewrite them
as a single value having the same semantics".