Expose scenario states (#168)

* Scenarios (WIP)

* Update ToJson and StatefulBehaviorTests

* fix tests

* Update comment

src/WireMock.Net/Admin/Mappings/MappingModel.cs

@@ -30,13 +30,13 @@ namespace WireMock.Admin.Mappings
         /// <summary>
         /// Execution state condition for the current mapping.
         /// </summary>
-        public object WhenStateIs { get; set; }
+        public string WhenStateIs { get; set; }
 
         /// <summary>
         /// The next state which will be signaled after the current mapping execution.
         /// In case the value is null state will not be changed.
         /// </summary>
-        public object SetStateTo { get; set; }
+        public string SetStateTo { get; set; }
 
         /// <summary>
         /// The request.