The validation rule is described in
http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-02#section-4.1.2

Instead of emitting common headers on decoder side, encoder
now keeps track of removed common headers and encode it
as necessary.

Following cases are now treated as connection error of type
PROTOCOL_ERROR.

* Receiving HEADERS from client in reserved (local)
* Receiving PUSH_PROMISE against peer-initiated stream
* Receiving WINDOW_UPDATE against in reserved (local, remote)

If multiple same ID are found, use the last one.

If multiple required headers (e.g., :path) found, return HTTP 400
error.
Fix util::strieq(a,b,n) where boundary of b is not checked in the
loop.

This may be useful to the misbehaving implementation, but it could
result in lots of RST_STREAM, so just ignore it for now.