net/http: revert CL 89275 (don't sniff Content-Type when nosniff set)

net/http: revert CL 89275 (don't sniff Content-Type when nosniff set)

Also updates the bundled http2 to x/net/http2 git rev 49c15d80 for:

   http2: revert CL 107295 (don't sniff Content-type in Server when nosniff)
   https://golang.org/cl/126895

Fixes #24795

Change-Id: I6ae1a21c919947089274e816eb628d20490f83ce
Reviewed-on: https://go-review.googlesource.com/126896
Reviewed-by: Damien Neil <dneil@google.com>