If the Transport got a stream error on the response headers, it was
never unblocking the client. Previously, Response.Body reads would be
aborted with the stream error, but RoundTrip itself would never
unblock.

The Transport now also sends a RST_STREAM to the server when we
encounter a stream error.

Also, add a "Cause" field to StreamError with additional detail. The
old code was just returning the detail, without the stream error
header.

Fixes golang/go#16572

Change-Id: Ibecedb5779f17bf98c32787b68eb8a9b850833b3
Reviewed-on: https://go-review.googlesource.com/25402
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>