added Addr() to test server

Signed-off-by: Alex Suraci <asuraci@pivotal.io>

ghttp/test_server.go

diff --git a/ghttp/test_server.go b/ghttp/test_server.go
index 1e5afc8..4bbb2e4 100644
--- a/ghttp/test_server.go
+++ b/ghttp/test_server.go
@@ -182,6 +182,11 @@
 	return s.HTTPTestServer.URL
 }
 
+//Addr() returns the address on which the server is listening.
+func (s *Server) Addr() string {
+	return s.HTTPTestServer.Listener.Addr().String()
+}
+
 //Close() should be called at the end of each test.  It spins down and cleans up the test server.
 func (s *Server) Close() {
 	s.writeLock.Lock()