Alternate-Protocol is a hop-by-hop header

Alternate-Protocol is a hop-by-hop header

Google are pushing Alternate-Protocol header to arbitrarily move clients
from HTTP to their experimental and proprietary protocols.

When relayed through a proxy this can cause traffic bifurcation with
side effects ranging from client visible failures, to traffic increase
on the network as proxy caching is bypassed, to security vulnerabilities
or privacy information leaks over the experimental protocol.

Treating the Alternate-Protocol header as a hop-by-hop header only
relevant on client->origin connections we can both prevent the above
issues occuring on networks utilizing a proxy, and also participate in
the protocol exprimentation process ourselves.