Handle null attributes correctly

Author: Lee Richmond

src/attribute.ts

@@ -44,7 +44,7 @@ export default class Attribute {
       },
 
       set(value) : void {
-        if (!value.hasOwnProperty('isAttr')) {
+        if (!value || !value.hasOwnProperty('isAttr')) {
           attr.setter(this, value);
         }
       }

test/fixtures.ts

@@ -18,6 +18,8 @@ let Author = Person.extend({
     jsonapiType: 'authors'
   },
 
+  nilly:      attr(),
+
   multiWords: hasMany('multi_words'),
   books:      hasMany(),
   tags:       hasMany(),

test/unit/model-test.ts

@@ -14,7 +14,8 @@ describe('Model', function() {
         type: 'authors',
         attributes: {
           firstName: 'Donald Budge',
-          unknown: 'adsf'
+          unknown: 'adsf',
+          nilly: null
         },
         relationships: {
           unknownrelationship: {
@@ -107,7 +108,8 @@ describe('Model', function() {
       let instance = Model.fromJsonapi(doc.data, doc);
       expect(instance.firstName).to.eq('Donald Budge');
       expect(instance.attributes).to.eql({
-        firstName: 'Donald Budge'
+        firstName: 'Donald Budge',
+        nilly: null
       })
     });
 
@@ -126,6 +128,11 @@ describe('Model', function() {
       expect(instance).to.not.have.property('unknownrelationship');
     });
 
+    it('does not blow up on null attributes', function() {
+      let instance = Model.fromJsonapi(doc.data, doc);
+      expect(instance.nilly).to.eq(null);
+    });
+
     it('assigns metadata correctly', function() {
       let instance = Model.fromJsonapi(doc.data, doc);
       expect(instance.__meta__).to.eql({