Edited Post.java to handle concers about copying and nulls

Maddalab suggested (https://github.com/tumblr/jumblr/pull/21/files#r5741882) that because I didn't actually use the reflection, it would be better to use System.arrayCopy. jay-to-the-dee suggested (https://github.com/tumblr/jumblr/pull/21/files#r5763924) that when notes is null, it would throw a NullPointerException. I believe I have fixed both of these issues.