x/net/http2: ignore unknown 1xx responses like HTTP/1

[bradfitz]

We should do the same as #17739 but for http2.Transport, so HTTP/1 and HTTP/2 are in sync behavior-wise for Go 1.11.

[gopherbot]

Change https://golang.org/cl/123615 mentions this issue: http2: ignore unknown 1xx responses like HTTP/1

[gopherbot]

Change https://golang.org/cl/123675 mentions this issue: http2: fix bug in earlier CL 123615

[gopherbot]

Change https://golang.org/cl/123676 mentions this issue: net/http: update bundled http2