mime/multipart: remove newline at top of the multipart.

R=golang-dev, bradfitz, arctanofyourface
CC=golang-dev
https://golang.org/cl/4635063

src/pkg/mime/multipart/writer.go

@@ -61,7 +61,11 @@ func (w *Writer) CreatePart(header textproto.MIMEHeader) (io.Writer, os.Error) {
 		}
 	}
 	var b bytes.Buffer
+	if w.lastpart != nil {
 		fmt.Fprintf(&b, "\r\n--%s\r\n", w.boundary)
+	} else {
+		fmt.Fprintf(&b, "--%s\r\n", w.boundary)
+	}
 	// TODO(bradfitz): move this to textproto.MimeHeader.Write(w), have it sort
 	// and clean, like http.Header.Write(w) does.
 	for k, vv := range header {

src/pkg/mime/multipart/writer_test.go

@@ -30,6 +30,13 @@ func TestWriter(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Close: %v", err)
 		}
+		s := b.String()
+		if len(s) == 0 {
+			t.Fatal("String: unexpected empty result")
+		}
+		if s[0] == '\r' || s[0] == '\n' {
+			t.Fatal("String: unexpected newline")
+		}
 	}
 
 	r := NewReader(&b, w.Boundary())