Fix handling of 'registration' in State API

* Fixes existing test which was failing on now conformant LRS
* Make 'registration' nullable in StateDocument
* Add setting registration value in query params and properties for
  certain State requests which should have already had it

Author: brianjmiller

TinCan/Documents/StateDocument.cs

@@ -21,6 +21,6 @@ public class StateDocument : Document
     {
         public Activity activity { get; set; }
         public Agent agent { get; set; }
-        public Guid registration { get; set; }
+        public Nullable<Guid> registration { get; set; }
     }
 }

TinCan/RemoteLRS.cs

@@ -552,6 +552,12 @@ public StateLRSResponse RetrieveState(String id, Activity activity, Agent agent,
             state.activity = activity;
             state.agent = agent;
 
+            if (registration != null)
+            {
+                queryParams.Add("registration", registration.ToString());
+                state.registration = registration;
+            }
+
             var resp = GetDocument("activities/state", queryParams, state);
             if (resp.status != HttpStatusCode.OK && resp.status != HttpStatusCode.NotFound)
             {
@@ -570,6 +576,10 @@ public LRSResponse SaveState(StateDocument state)
             queryParams.Add("stateId", state.id);
             queryParams.Add("activityId", state.activity.id.ToString());
             queryParams.Add("agent", state.agent.ToJSON(version));
+            if (state.registration != null)
+            {
+                queryParams.Add("registration", state.registration.ToString());
+            }
 
             return SaveDocument("activities/state", queryParams, state);
         }