zstd: pooledZipWriter should return Writers to the same pool (#426)

This code was previously Get'ing objects from one pool and later
Put'ing them to a different pool, so they were never reused.

zstd/zip.go

@@ -64,8 +64,9 @@ func (r *pooledZipReader) Close() error {
 }
 
 type pooledZipWriter struct {
-	mu  sync.Mutex // guards Close and Read
-	enc *Encoder
+	mu   sync.Mutex // guards Close and Read
+	enc  *Encoder
+	pool *sync.Pool
 }
 
 func (w *pooledZipWriter) Write(p []byte) (n int, err error) {
@@ -83,7 +84,7 @@ func (w *pooledZipWriter) Close() error {
 	var err error
 	if w.enc != nil {
 		err = w.enc.Close()
-		zipReaderPool.Put(w.enc)
+		w.pool.Put(w.enc)
 		w.enc = nil
 	}
 	return err
@@ -104,7 +105,7 @@ func ZipCompressor(opts ...EOption) func(w io.Writer) (io.WriteCloser, error) {
 				return nil, err
 			}
 		}
-		return &pooledZipWriter{enc: enc}, nil
+		return &pooledZipWriter{enc: enc, pool: &pool}, nil
 	}
 }