Fixed setter for response transformer

Author: JanHolger

src/main/java/org/javawebstack/httpclient/HTTPClient.java

@@ -89,7 +89,7 @@ public HTTPClient setBaseUrl(String baseUrl) {
     }
 
     public HTTPClient transformer(ResponseTransformer responseTransformer){
-        responseTransformer = responseTransformer;
+        this.responseTransformer = responseTransformer;
         return this;
     }