This fixes the problem that existed with received headers:
The server sends some headers ("set-cookie") more than once
in the same reply, which cannot be stored in std::map.
The old code just ignored the additional cookies, while
curlthreading3 (since the introduction of AIHTTPHeaders)
caused an assertion.
AIHTTPReceivedHeaders is written around a std::multimap
and allows to retrieve multiple headers with the same key.
Also, it is case insensitive so that if a server sends
"Content-Type" it will still find it (the viewer looks for
"content-type").
