Handle multiple header values in SPDY

SPDY sends multiple header values for the same header key by null-byte
separating them.

This patch maps the multiple values the same way
qnetworkreplyhttpimpl.cpp
does. With this patch applied we can now log on to GMail using SPDY.

Change-Id: I03656ad1695d13b5c3ed252794dc6c89c67c7b97
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>