http: reply to Expect 100-continue requests automatically

http: reply to Expect 100-continue requests automatically

This CL replaces my earlier https://golang.org/cl/1640044/show
in which Continue handling was explicit.  Instead, this CL makes
it automatic.  Reading from Body() is an implicit acknowledgement
that the request headers were fine and the body is wanted.  In that
case, the 100 Continue response is written automatically when the
request continues the "Expect: 100-continue" header.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/1610042