fixing an issue with PopulateMergeMatch giving errors on db rebuild

I was experiencing som issues using the `PopulateMergeMatch` - it basically didn't let me use it, and once I fixed that (see line 20), it started deleting my entries, which I don't believe should be the purpose, hence i introduced `$mode` in line 35.
I furthermore had to unset `PopulateMergeMatch`, as the object couldn't be saved with an array (line 56).

I don't know if you can reproduce the issue? I'll use this branch for now as it works for me like this.

This is the yml I'm using:

```
VermarktungsartPrototype:
  KAUF:
    Title: "Kauf"
    Identifier: "KAUF"
    PopulateMergeMatch:
      - Identifier
  MIETE_PACHT:
    Title: "Miete/Pacht"
    Identifier: "MIETE_PACHT"
    PopulateMergeMatch:
      - Identifier
```