cmd/godoc: set Content-Type in fmt handler

Change-Id: I51b12838c65d67d4323e07d70f7c1e66fce23251
Reviewed-on: https://go-review.googlesource.com/14938
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/handlers.go

@@ -141,5 +141,6 @@ func fmtHandler(w http.ResponseWriter, r *http.Request) {
 	} else {
 		resp.Body = string(body)
 	}
+	w.Header().Set("Content-type", "application/json; charset=utf-8")
 	json.NewEncoder(w).Encode(resp)
 }