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 <bradfitz@golang.org>
    Reviewed-by: Chris Broadfoot <cbro@golang.org>