Conditions can fail when additional entities are used

I'm using my new ContributionSoft trigger, but this problem is common to any trigger that loads an additional entity via API.

CRM_Civirules_Trigger_Post::getTriggerDataFromPost() refers to fields by their internal name. However, data retrieved via API is returned using the uniquename for the fields. E.g. if you have a Field Value Comparison on a contact's source field, it will be stored as, e.g., source = 4. This works if your trigger data is populated from hook_civicrm_post. However, if it's populated via API (as "additional" entities seem to be), the field will be named contact_source. Additionally, the API will not return all fields by default.

I think I have a solution in mind; if I get it working I'll submit a PR.