Fix a bug in SPDY plus HTTP caching.

With spdy/3 the built-in headers were renamed from 'status'
to ':status' and 'version' to ':version'. This caused the
response cache to break, since it was parsing lines like
':status: 200 OK' by splitting on the first colon.

The fix drops these synthetic headers as soon as they're
received; converting them to the HTTP form immediately.