Don't close idle connections when request body cannot be obtained

It's an unrelated error, no point in closing keep-alive connections.

client.go

@@ -564,8 +564,7 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		if req.body != nil {
 			body, err := req.body()
 			if err != nil {
-				c.HTTPClient.CloseIdleConnections()
-				return resp, err
+				return nil, err
 			}
 			if c, ok := body.(io.ReadCloser); ok {
 				req.Body = c