net/http: Show more data for unsolicited response

net/http: Show more data for unsolicited response

Show more than one character when we recieve a unsolicited
response on an idle HTTP channel. Showing more than one
byte is really useful when you want to debug your program
when you get this message.

Change-Id: I3caf9f06420e7c2a2de3e4eb302c5dab95428fdb
Reviewed-on: https://go-review.googlesource.com/13959
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>