Update client.go to get response header whether there is an error or not. (#751)

Update client.go to get response header whether there is an error or not. Because 429 Too Many Requests error response has "Retry-After" header.

Author: ando-masaki

client.go

@@ -129,14 +129,14 @@ func (c *Client) sendRequest(req *http.Request, v Response) error {
 
 	defer res.Body.Close()
 
-	if isFailureStatusCode(res) {
-		return c.handleErrorResp(res)
-	}
-
 	if v != nil {
 		v.SetHeader(res.Header)
 	}
 
+	if isFailureStatusCode(res) {
+		return c.handleErrorResp(res)
+	}
+
 	return decodeResponse(res.Body, v)
 }