http: disable sniffer for now.

Something is broken, and investigation is underway.
In the meantime, godoc is broken, so disable sniffing for now
by reverting to the pre-sniffer state.

R=r
CC=golang-dev
https://golang.org/cl/4809046

src/pkg/http/server.go

@@ -255,7 +255,9 @@ func (w *response) WriteHeader(code int) {
 	} else {
 		// If no content type, apply sniffing algorithm to body.
 		if w.header.Get("Content-Type") == "" {
-			w.needSniff = true
+			// NOTE(dsymonds): the sniffing mechanism in this file is currently broken.
+			//w.needSniff = true
+			w.header.Set("Content-Type", "text/html; charset=utf-8")
 		}
 	}