httptest: default the Recorder status code to 200 on a Write

This matches the real ResponseWriter behavior.

R=rsc
CC=golang-dev
https://golang.org/cl/4291048
diff --git a/src/pkg/http/httptest/recorder.go b/src/pkg/http/httptest/recorder.go
index 8d70c28..0dd19a6 100644
--- a/src/pkg/http/httptest/recorder.go
+++ b/src/pkg/http/httptest/recorder.go
@@ -42,6 +42,9 @@
 	if rw.Body != nil {
 		rw.Body.Write(buf)
 	}
+	if rw.Code == 0 {
+		rw.Code = http.StatusOK
+	}
 	return len(buf), nil
 }