Populate taxonomy fields with data from existing posts

[0xLBF]

Using the Save To Post Type processor, fields can be configured to get saved as taxonomy terms assigned to a post, however when editing an existing post the field doesn't seem to get populated with those saved terms. This patch fixes that, but let me know if there's a simpler way to get it to work.

[New0]

Thank you @0xLBF
@Shelob9 I'm not sure we should change default behaviour for editing posts at that point. Maybe we could work on a feature request to choose the behavior when editing posts. This would let the user choose if (s)he wants to add terms and keep saved terms or reset terms as this is currently the case.