gddo-server: do not redirect plain text requests

Pkg.go.dev doesn't support an plain text interface yet, but godoc.org
does: https://github.com/golang/gddo/wiki/Plain-Text-Interface

Do not redirect these requests until a decision is made about that
feature.

For golang/go#44029

Change-Id: I54466f2180db3b30a24299e49fc8fca107fe9f9d
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/288949
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>

gddo-server/pkgsite.go

diff --git a/gddo-server/pkgsite.go b/gddo-server/pkgsite.go
index 6744857..52c477d 100644
--- a/gddo-server/pkgsite.go
+++ b/gddo-server/pkgsite.go
@@ -249,6 +249,10 @@
 		return false
 	}
 
+	if r.Header.Get("Accept") == "text/plain" {
+		return false
+	}
+
 	// Requests to api.godoc.org and talks.godoc.org are not redirected.
 	if strings.HasPrefix(r.URL.Host, "api") || strings.HasPrefix(r.URL.Host, "talks") {
 		return false