soup-cache: update state of SoupMessages when returning data from the cache

soup-cache: update state of SoupMessages when returning data from the cache

SoupCache generates conditional requests to validate some resources. As the
cache is transparent for the client it must update the state of the
SoupMessage used by the SoupHTTPRequest as if the message was really
transmitted over the network. SoupCache was not updating the state
(basically status code and headers) of the SoupMessage that belongs to the
SoupHTTPRequest but the one used for freshness validation.

https://bugzilla.gnome.org/show_bug.cgi?id=649965