Updated doc examples to open using read binary mode
Docs update and canonical url fix
fix for nested MapFields

When using nested MapFields from a document loaded from the database, the
nested dictionaries aren't converted to BaseDict, so changes aren't
marked.

This also includes a change when marking a field as changed to ensure that
nested fields aren't included in a $set query if a parent is already marked
as changed. Not sure if this could occur but it prevents breakage if it does.