fix(go-generator): optional payload results in error when it's not set (#154)

Author: marceljk

templates/go/client.mustache

@@ -307,7 +307,7 @@ func (c *APIClient) prepareRequest(
 	var body *bytes.Buffer
 
 	// Detect postBody type and post.
-	if postBody != nil {
+	if !IsNil(postBody) {
 		contentType := headerParams["Content-Type"]
 		if contentType == "" {
 			contentType = detectContentType(postBody)