net/http: update bundled http2

Update x/net/http2 to git rev 6b17b9baf5 for:

   http2: stop rejecting outgoing paths beginning with two slashes
   https://golang.org/cl/45773

This also uses an updated version of x/tools/cmd/bundle (CL 45190)
that fixes an edge case where it used to drop some comments.

Updates #20627
Fixes #19103

Change-Id: I450d61485e66098f4f8a79954f729f7bcd85856f
Reviewed-on: https://go-review.googlesource.com/45700
Run-TryBot: Brad Fitzpatrick <[email protected]>
Reviewed-by: Chris Broadfoot <[email protected]>

Author: bradfitz