SPDY: consistently handle control frames with unknown type.

SPDY: consistently handle control frames with unknown type.

The SPDY draft 2 specification requires that if an endpoint receives a
control frame for a type it does not recognize, it must ignore the frame.
But the 3 and 3.1 drafts don't seem to declare any behavior for such case.
Then sticking with the previous draft in this matter looks to be right.

But previously, only 8 least significant bits of the type field were
parsed while the rest of 16 bits of the field were checked against zero.
Though there are no known frame types bigger than 255, this resulted in
inconsistency in handling of such frames: they were not recognized as
valid frames at all, and the connection was closed.