Hacks the action splitter in case we want to use __ as separator before the action separator.

We would like to do some magic like indexing django related field implicitly like in this demo mapping: https://github.com/novapost/django-esutils/blob/master/demo/demo_esutils/mappings.py#L13

Then the BaseMappingType do the job: https://github.com/novapost/django-esutils/blob/master/django_esutils/mappings.py#L98

But to do that, we need to hack the current `split_field_action` method (cf.: this PR). What do you think about that ? Or is there another way to do it ?