http: Support relative URL redirection

In RFC 2616, this was explicitly said to be an absolute URL,
while in an upcoming draft [1] it is allowed to be relative as well.

[1] http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-25#section-7.1.2



Signed-off-by: Martin Storsjö <martin@martin.st>