net/http: support for setting trailers from a server Handler

We already had client support for trailers, but no way for a server to
set them short of hijacking the connection.

Fixes #7759

Change-Id: Ic83976437739ec6c1acad5f209ed45e501dbb93a
Reviewed-on: https://go-review.googlesource.com/2157
Reviewed-by: Andrew Gerrand <adg@golang.org>