Writer: initialize the writer on Flush() if wasn't initialized before

This allows compressing empty files, because without this change
the proper headers were not emitted when reading an empty file
via e.g. io.Copy().

Signed-off-by: Nikolay Edigaryev <edigaryev@gmail.com>

writer.go

@@ -150,6 +150,10 @@ func (w *Writer) Flush() (err error) {
 	case writeState:
 	case errorState:
 		return w.state.err
+	case newState:
+		if err = w.init(); w.state.next(err) {
+			return
+		}
 	default:
 		return nil
 	}

writer_test.go

@@ -17,6 +17,7 @@ import (
 
 func TestWriter(t *testing.T) {
 	goldenFiles := []string{
+		"testdata/empty.txt",
 		"testdata/e.txt",
 		"testdata/gettysburg.txt",
 		"testdata/Mark.Twain-Tom.Sawyer.txt",
@@ -75,7 +76,7 @@ func TestWriter(t *testing.T) {
 					t.Errorf("invalid sizes: got %d; want %d", got, want)
 				}
 
-				if got, want := out.Bytes(), raw; !reflect.DeepEqual(got, want) {
+				if got, want := out.Bytes(), raw; !bytes.Equal(got, want) {
 					t.Fatal("uncompressed data does not match original")
 				}