Unable to find view at path 'App.xxx' when using DateTime in {{#each}}

`DateTime#unknownProperty` returns `null` if it doesn't match any known formatters, but this kills Ember's view lookup as it expects to get `undefined` for unknown properties.

I'm not 100% sure if this is an issue with DateTime, or whether Ember should also handle nulls.

Here's a fiddle which demonstrates the issue:

http://jsfiddle.net/rlivsey/aKHhr/
