[release-branch.go1.7] net/http: fix unwanted HTTP/2 conn Transport crash after IdleConnTimeout

Go 1.7 crashed after Transport.IdleConnTimeout if an HTTP/2 connection
was established but but its caller no longer wanted it. (Assuming the
connection cache was enabled, which it is by default)

Fixes #16208

Change-Id: I9628757f7669e344f416927c77f00ed3864839e3
Reviewed-on: https://go-review.googlesource.com/27450


Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/28637


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>