Use respond_to? everywhere we use method_missing

[smtlaissezfaire]

All of the method_missing stuff is confusing (and doesn't perform well). We should also be overriding respond_to? anytime we override method_missing

[cheald]

method_missing is used to provide access to dynamic keys on documents (ie, in the case when a key is present in a document, but wasn't declared on the model directly). How would respond_to? be helpful here?