isKnown - info object can be undefined (#72)

Add guard to prevent the evaluation of 'known' for linked relationships when the relationship key is not being checked

addon/model-ext.js

@@ -142,7 +142,7 @@ Model.reopen({
   },
 
   observeUnknownRelationshipLoaded(sender, key/*, value, rev*/) {
-    if (Tracker.trackingIsSetup(this)) {
+    if (Tracker.trackingIsSetup(this) && Tracker.isTracking(this, key)) {
       let saved = Tracker.saveLoadedRelationship(this, key);
       if (saved) {
         this.removeObserver(key, this, 'observeUnknownRelationshipLoaded');

addon/tracker.js

@@ -126,6 +126,18 @@ export default class Tracker {
     return info;
   }
 
+  /**
+   * Find whether this key is currently being tracked.
+   *
+   * @param {DS.Model} model
+   * @param {string} [key]
+   * @returns {boolean} true if this key is being tracked. false otherwise
+   */
+  static isTracking(model, key) {
+    let info = (model.constructor.trackerKeys || {});
+    return !!info[key];
+  }
+
   /**
    * On the model you can set options like:
    *