• durran introduced a bug for typecasting for BigDecimal & Time in this
  commit: 42ae841fdb91d61 which prevents a field's value from being
  typecasted after it has been retrieved from the database.

The change assumes that the value has already been cast to its appropriate
value; however, BigDecimal & Date are not sufficiently represented in
MongoDB and need to be cast after the document has been instantiated.

• this change proposes that certain field types should always be
  typecast (BigDecimal & Date).