Make stdcopy.stdWriter goroutine safe.

Stop using global variables as prefixes to inject the writer header.
That can cause issues when two writers set the length of the buffer in
the same header concurrently.

Stop Writing to the internal buffer twice for each write. This could
mess up with the ordering information is written.

Signed-off-by: David Calavera <david.calavera@gmail.com>

stdcopy/stdcopy_test.go

@@ -17,10 +17,9 @@ func TestNewStdWriter(t *testing.T) {
 }
 
 func TestWriteWithUnitializedStdWriter(t *testing.T) {
-	writer := StdWriter{
-		Writer:  nil,
-		prefix:  Stdout,
-		sizeBuf: make([]byte, 4),
+	writer := stdWriter{
+		Writer: nil,
+		prefix: byte(Stdout),
 	}
 	n, err := writer.Write([]byte("Something here"))
 	if n != 0 || err == nil {
@@ -180,7 +179,7 @@ func TestStdCopyDetectsCorruptedFrame(t *testing.T) {
 		src:          buffer}
 	written, err := StdCopy(ioutil.Discard, ioutil.Discard, reader)
 	if written != startingBufLen {
-		t.Fatalf("Expected 0 bytes read, got %d", written)
+		t.Fatalf("Expected %d bytes read, got %d", startingBufLen, written)
 	}
 	if err != nil {
 		t.Fatal("Didn't get nil error")