…efault value is provided in `default=<value>` or as a `default_factory=copy_value(<value>)`, this is done only **once per field**, to accelerate future access. If the value was converted on the way, the converted value is used to replace the default value, or the default value copied by the factory. Fixed [#57](#57)

# Conflicts:
#	pyfields/core.py
#	pyfields/helpers.py