Specify blog post accessors

Once again, rather than fixing the acceptance error directly, we drop down to the model test level to reproduce the error. We specify that we want a `title` field--as well as a `body` field, since we know we'll need that and it feels safe to go ahead and add it. The behavior we want is that we want those fields to be `nil` for a new model instance.

Inner red: undefined method `title' for #<BlogPost id: nil>

With this specification, we've reproduced the error from the acceptance test.

spec/models/blog_post_spec.rb

@@ -6,4 +6,11 @@
     expect{ blog_post = BlogPost.new }.not_to raise_error
   end
 
+  it "defaults fields to nil" do
+    blog_post = BlogPost.new
+
+    expect(blog_post.title).to be_nil
+    expect(blog_post.body).to be_nil
+  end
+
 end