The net/http Transport was testing for a sentinel x/net/http2 error
value with ==, which meant it was only testing the bundled version. If
a user enabled http2 via golang.org/x/net/http2, the error value had a
different name.

This also updates the bundled x/net/http2 to git rev ab555f36 for:

    http2: add internal function isNoCachedConnError to test for ErrNoCachedConn
    https://golang.org/cl/87297

Fixes #22091

Change-Id: I3fb85e2b7ba7d145dd66767e1795a56de633958c
Reviewed-on: https://go-review.googlesource.com/87298Reviewed-by: Tom Bergan <tombergan@google.com>