Use http code to check object in lookaside cache
    
    Currently, it only checks the size of the http object returned but it
    may happen that the size of the 404 error page match the size of the
    tarball. You may think this is a corner case we'll never hit... well, we
    hit it, :)
    
    This patch it's adding a check based on the http code, so to give it as
    already uploaded we need it to return 200 code and match the actual
    size.