http2: set default User-Agent if not otherwise specified
http2: set default User-Agent if not otherwise specified

Tests in main repo, named:

=== RUN   TestTransportUserAgent_h1
--- PASS: TestTransportUserAgent_h1 (0.00s)
=== RUN   TestTransportUserAgent_h2
--- PASS: TestTransportUserAgent_h2 (0.03s)

(in upcoming CL)

Updates golang/go#13685

Change-Id: I2b62489eb03b7ff791f1f6ae6dc9c4e4f6895d39
Reviewed-on: https://go-review.googlesource.com/18285
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cmars/tools,golang/net,cmars/tools,cesanta/goxnet,cesanta/goxnet,albertjin/golang-net,cmars/tools,DramaFever/golang.org-x-net,cmars/tools,DramaFever/golang.org-x-net,albertjin/golang-net