not panic if server closed multiple times

Signed-off-by: Jatin Naik <jnaik@pivotal.io>

ghttp/test_server.go

diff --git a/ghttp/test_server.go b/ghttp/test_server.go
index 2f76bb2..093550d 100644
--- a/ghttp/test_server.go
+++ b/ghttp/test_server.go
@@ -202,7 +202,9 @@
 
 	server := s.HTTPTestServer
 	s.HTTPTestServer = nil
-	server.Close()
+	if server != nil {
+		server.Close()
+	}
 }
 
 //ServeHTTP() makes Server an http.Handler

ghttp/test_server_test.go

diff --git a/ghttp/test_server_test.go b/ghttp/test_server_test.go
index b8a9318..88b3246 100644
--- a/ghttp/test_server_test.go
+++ b/ghttp/test_server_test.go
@@ -78,6 +78,13 @@
 		})
 	})
 
+	Describe("closing server mulitple times", func() {
+		It("should not fail", func() {
+			s.Close()
+			Ω(s.Close).ShouldNot(Panic())
+		})
+	})
+
 	Describe("allowing unhandled requests", func() {
 		Context("when true", func() {
 			BeforeEach(func() {