added regression test to be sure that ValidationFeature can be called…

… twice without problem

tests/ServiceStack.WebHost.Endpoints.Tests/CustomerServiceValidationTests.cs

@@ -195,6 +195,14 @@ public void SetUp()
 				Postcode = "11215",
 			};
 		}
+
+        [Test]
+        public void ValidationFeature_add_request_filter_once()
+        {
+            var old = appHost.RequestFilters.Count; 
+            appHost.LoadPlugin(new ValidationFeature());
+            Assert.That(old, Is.EqualTo(appHost.RequestFilters.Count));
+        }
 
 		[Test]
 		public void Validates_ValidRequest_request_on_Post()