Observers for documents don't work as expected

[ryanwtyler]

https://github.com/GoogleWebComponents/firebase-element/blob/master/firebase-document.html#L85

this line calls set on entire document instead of subproperty that changed. this causes the change record's path to be "object" instead of "object.subprop". my app has things that depend on the value of "subprop" and I can't set up observers for these dependencies.