mime/multipart: add Writer data race test

Camlistore uses this pattern to do streaming writes, as do
others I imagine, and it was broken by the lazy boundary
change.

LGTM=dvyukov, ruiu
R=ruiu, dvyukov
CC=golang-codereviews, mathieu.lonjaret
https://golang.org/cl/116690043

src/pkg/mime/multipart/writer_test.go

@@ -111,3 +111,15 @@ func TestWriterSetBoundary(t *testing.T) {
 		t.Errorf("expected my-separator in output. got: %q", got)
 	}
 }
+
+func TestWriterBoundaryGoroutines(t *testing.T) {
+	// Verify there's no data race accessing any lazy boundary if it's used by
+	// different goroutines. This was previously broken by
+	// https://codereview.appspot.com/95760043/ and reverted in
+	// https://codereview.appspot.com/117600043/
+	w := NewWriter(ioutil.Discard)
+	go func() {
+		w.CreateFormField("foo")
+	}()
+	w.Boundary()
+}