Add nested method to help with document mapping
Add nested method to help with document mapping

cad46aa introduced a much cleaner update system. There are a couple of places the code could have been a bit cleaner, though.

The nested `map_field_names_and_values()` method addressed one of those. `_update()` can now handle mapping field names and getting values from the internal document with a single line of code (or two in the case of an atomic update).

dirn/Simon,dirn/Simon