BaseConditionChoice.default_value_field is allowed to be a function,

but BaseConditionOperator.value_field wasn't handling this case,

breaking some operators/choices. It now checks if it's a function

and calls it.
BaseConditionOperator.has_custom_value_field has been reworked

accordingly, checking if the definitions on the class stray from the

defaults for operators, rather than checking the results of the

properties/functions being called. Without this fix, an operator could

be using the choice's default, but it wouldn't look like it, as it'd

get two different instances during the comparison.
A unit test has been added for these changes.

Unit tests pass.