Fix role mapping integration test

Both the built-in and custom source serializer will always serialize null values within the role mapping metadata

Author: russcam

src/Tests/XPack/Security/RoleMapping/RoleMappingCrudTests.cs

@@ -135,11 +135,7 @@ protected override void ExpectAfterCreate(IGetRoleMappingResponse response)
 
 			mapping.Enabled.Should().BeFalse();
 			mapping.Roles.Should().BeEquivalentTo("admin");
-			//source serializer includes nulls on deserialize
-			if (TestClient.Configuration.UsingCustomSourceSerializer)
-				mapping.Metadata.Should().HaveCount(2).And.ContainKeys("x", "z");
-			else
-				mapping.Metadata.Should().HaveCount(1).And.ContainKeys("x");
+			mapping.Metadata.Should().HaveCount(2).And.ContainKeys("x", "z");
 			mapping.Rules.Should().NotBeNull();
 
 			var allMapping = mapping.Rules as AllRoleMappingRule;