net/http: updated bundled http2 to finish trailer support

This updates the bundled copy of x/net/http2 to git rev d2ecd08
for https://golang.org/cl/17912 (http2: send client trailers)
and enables the final Trailer test for http2.

Fixes #13557

Change-Id: Iaa15552b82bf7a2cb01b7787a2e1ec5ee680a9d3
Reviewed-on: https://go-review.googlesource.com/17935
Run-TryBot: Brad Fitzpatrick <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: bradfitz

src/net/http/clientserver_test.go

@@ -469,10 +469,7 @@ func testCancelRequestMidBody(t *testing.T, h2 bool) {
 
 // Tests that clients can send trailers to a server and that the server can read them.
 func TestTrailersClientToServer_h1(t *testing.T) { testTrailersClientToServer(t, h1Mode) }
-func TestTrailersClientToServer_h2(t *testing.T) {
-	t.Skip("skipping in http2 mode; golang.org/issue/13557")
-	testTrailersClientToServer(t, h2Mode)
-}
+func TestTrailersClientToServer_h2(t *testing.T) { testTrailersClientToServer(t, h2Mode) }
 
 func testTrailersClientToServer(t *testing.T, h2 bool) {
 	defer afterTest(t)