Improve 204 no content handling (#654)

* set request error to nil upon 204 No Content response

* fix empty body check (ioutil.ReadAll will return an empty slice rather than nil https://go.googlesource.com/go/+/refs/tags/go1.16/src/io/io.go#627)

middleware.go

@@ -334,6 +334,7 @@ func responseLogger(c *Client, res *Response) error {
 
 func parseResponseBody(c *Client, res *Response) (err error) {
 	if res.StatusCode() == http.StatusNoContent {
+		res.Request.Error = nil
 		return
 	}
 	// Handles only JSON or XML content type

response.go

@@ -93,7 +93,7 @@ func (r *Response) Cookies() []*http.Cookie {
 
 // String method returns the body of the server response as String.
 func (r *Response) String() string {
-	if r.body == nil {
+	if len(r.body) == 0 {
 		return ""
 	}
 	return strings.TrimSpace(string(r.body))
@@ -156,7 +156,7 @@ func (r *Response) setReceivedAt() {
 }
 
 func (r *Response) fmtBodyString(sl int64) string {
-	if r.body != nil {
+	if len(r.body) > 0 {
 		if int64(len(r.body)) > sl {
 			return fmt.Sprintf("***** RESPONSE TOO LARGE (size - %d) *****", len(r.body))
 		}