http: Add a default instance to UrlFetchTransport

https://codereview.appspot.com/12666043/

Author: ngmiceli-g

google-http-client-appengine/src/main/java/com/google/api/client/extensions/appengine/http/UrlFetchTransport.java

@@ -16,6 +16,7 @@
 
 import com.google.api.client.http.HttpMethods;
 import com.google.api.client.http.HttpTransport;
+import com.google.api.client.util.Beta;
 import com.google.api.client.util.Preconditions;
 import com.google.appengine.api.urlfetch.FetchOptions;
 import com.google.appengine.api.urlfetch.HTTPMethod;
@@ -88,6 +89,23 @@ public UrlFetchTransport() {
     certificateValidationBehavior = builder.certificateValidationBehavior;
   }
 
+  /**
+   * {@link Beta} <br/>
+   * Returns a global thread-safe instance.
+   *
+   * @since 1.17
+   */
+  @Beta
+  public static UrlFetchTransport getDefaultInstance() {
+    return InstanceHolder.INSTANCE;
+  }
+
+  /** Holder for the result of {@link #getDefaultInstance()}. */
+  @Beta
+  static class InstanceHolder {
+    static final UrlFetchTransport INSTANCE = new UrlFetchTransport();
+  }
+
   @Override
   public boolean supportsMethod(String method) {
     return Arrays.binarySearch(SUPPORTED_METHODS, method) >= 0;