Fix PropertyPathMapper reference handling

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Actually I am not entirely sure, if this should get treated as "BC-break". 

Scalar values were treated by-reference before as well. This means for scalar values, when the value before and after are the same, the setter was never called.