Workaround for ED issue #3760

Temp workaround for ED issue emberjs/data#3760 until
PR emberjs/data#3765 is merged.

Change class structure also to reflect the new one introduced earlier by ED 1.13
(with dedicated `_shouldSerializeHasMany`).

addon/serializers/localforage.js

@@ -5,25 +5,35 @@ export default DS.JSONSerializer.extend({
 
   isNewSerializerAPI: true,
 
-  serializeHasMany: function (snapshot, json, relationship) {
-    var key = relationship.key;
-
-    if (this._canSerialize(key)) {
-      var payloadKey;
+  _shouldSerializeHasMany: function (snapshot, key, relationship) {
+    var relationshipType = snapshot.type.determineRelationshipType(relationship, this.store);
+    if (this._mustSerialize(key)) {
+      return true;
+    }
+    return this._canSerialize(key) &&
+      (relationshipType === 'manyToNone' ||
+        relationshipType === 'manyToMany' ||
+        relationshipType === 'manyToOne');
+  },  
 
-      // if provided, use the mapping provided by `attrs` in
-      // the serializer
-      payloadKey = this._getMappedKey(key);
-      if (payloadKey === key && this.keyForRelationship) {
-        payloadKey = this.keyForRelationship(key, "hasMany", "serialize");
-      }
+  // Omit the unknown hasMany relationships of pushed record
+  // (see https://github.com/emberjs/data/issues/3760)
+  // TODO: this override will be unecessary after merge of the following PR:
+  // https://github.com/emberjs/data/pull/3765
+  serializeHasMany: function(snapshot, json, relationship) {
+    var key = relationship.key;
 
-      var relationshipType = snapshot.type.determineRelationshipType(relationship, this.store);
+    if (this._shouldSerializeHasMany(snapshot, key, relationship)) {
+      var hasMany = snapshot.hasMany(key, { ids: true });
+      if (hasMany !== undefined) {
+        // if provided, use the mapping provided by `attrs` in
+        // the serializer
+        var payloadKey = this._getMappedKey(key);
+        if (payloadKey === key && this.keyForRelationship) {
+          payloadKey = this.keyForRelationship(key, "hasMany", "serialize");
+        }
 
-      if (relationshipType === 'manyToNone' ||
-        relationshipType === 'manyToMany' ||
-        relationshipType === 'manyToOne') {
-        json[payloadKey] = snapshot.hasMany(key, {ids: true});
+        json[payloadKey] = hasMany;
         // TODO support for polymorphic manyToNone and manyToMany relationships
       }
     }
@@ -91,6 +101,8 @@ export default DS.JSONSerializer.extend({
 
     // Remove the undefined hasMany relationships which will fail at normalization
     // (see https://github.com/emberjs/data/issues/3736)
+    // TODO: this block will be unecessary after merge of the following PR:
+    // https://github.com/emberjs/data/pull/3747
     var relationshipNames = Ember.get(primaryModelClass, 'relationshipNames');
     var relationships     = relationshipNames.hasMany;