cmd/godoc: do not use httptest
cmd/godoc: do not use httptest

httptest assumes it's running a test, it registers its own flags, which
means godoc ends up with mysterious flags.

By implement an http.ResponseWriter, we do not need to use httptest.

Fixes golang/go#28138

Change-Id: Ia0de8597c3edb0e7bdea6d8b3b2f1618a12f9239
Reviewed-on: https://go-review.googlesource.com/c/141417
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools