Minor: set request factory in Google http client constructor (#1509)

Co-authored-by: Dominique Villard <dominique.villard@orange.com>
Co-authored-by: Marvin Froeder <velo@users.noreply.github.com>

googlehttpclient/src/main/java/feign/googlehttpclient/GoogleHttpClient.java

@@ -1,5 +1,5 @@
 /**
- * Copyright 2012-2020 The Feign Authors
+ * Copyright 2012-2021 The Feign Authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -41,7 +41,6 @@ import feign.Response;
  * "https://api.github.com");
  */
 public class GoogleHttpClient implements Client {
-  private final HttpTransport transport;
   private final HttpRequestFactory requestFactory;
 
   public GoogleHttpClient() {
@@ -49,8 +48,11 @@ public class GoogleHttpClient implements Client {
   }
 
   public GoogleHttpClient(final HttpTransport transport) {
-    this.transport = transport;
-    this.requestFactory = transport.createRequestFactory();
+    this(transport.createRequestFactory());
+  }
+
+  public GoogleHttpClient(final HttpRequestFactory requestFactory) {
+    this.requestFactory = requestFactory;
   }
 
   @Override