fix: handle buffer encoding correctly for Go 1.16 compatibility

client.go

@@ -195,10 +195,23 @@ func (c *client) sendRequest(
 		}
 
 		if !c.contentEncoding.IsZero() {
-			body, err = c.encoder.Encode(body)
-			if err != nil {
-				return nil, internalError.WithErrCode(ErrCodeMarshalRequest,
-					fmt.Errorf("failed to marshal with json.Marshal: %w", err))
+			// Get the data from the buffer before encoding
+			var bufData []byte
+			if buf, ok := body.(*bytes.Buffer); ok {
+				bufData = buf.Bytes()
+				encodedBuf, err := c.encoder.Encode(bytes.NewReader(bufData))
+				if err != nil {
+					if buf, ok := body.(*bytes.Buffer); ok {
+						c.bufferPool.Put(buf)
+					}
+					return nil, internalError.WithErrCode(ErrCodeMarshalRequest,
+						fmt.Errorf("failed to encode request body: %w", err))
+				}
+				// Return the original buffer to the pool since we have a new one
+				if buf, ok := body.(*bytes.Buffer); ok {
+					c.bufferPool.Put(buf)
+				}
+				body = encodedBuf
 			}
 		}
 	}