In Go 1.6, the HTTP/1 client got Transport.ExpectContinueTimeout.

This makes the HTTP/2 client respect a Request's "Expect:
100-continue" field and the Transport.ExpectContinueTimeout
configuration.

This also makes sure to call the traceWroteRequest hook if the server
replied while we're still writing the request, since that code was
in the same spot and it couldn't be trivially separated.

Updates golang/go#13851 (fixed after integrating it into std)
Updates golang/go#15744

Change-Id: I67dfd68532daa6c4a0c026549c6e5cbfce50e1ea
Reviewed-on: https://go-review.googlesource.com/23235Reviewed-by: Andrew Gerrand <adg@golang.org>