net/http/httputil: don't panic in ReverseProxy unless running under a Server
Prior to the fix to #23643, the ReverseProxy didn't panic with
ErrAbortHandler when the copy to a client failed.
During Go 1.11 beta testing, we found plenty of code using
ReverseProxy in tests that were unprepared for a panic.
Change the behavior to only panic when running under the http.Server
that'll handle the panic.
Updates #23643
Change-Id: Ic1fa8405fd54c858ce8c797cec79d006833a9f7d
Reviewed-on: https://go-review.googlesource.com/122819 Reviewed-by: Ian Lance Taylor <iant@golang.org>