Write test for 'MetaProperty.propertyGenericType()'

[nipafx]

The method MetaProperty.propertyGenericType() is currently not tested. To resolve this issue meaningful tests should be provided for it. The corresponding ToDo should be removed.

Some Pointers

A short explanation of @jodastephen about propertyGenericTypes() can be found in this comment.

Tests can most likely be added to AbstractMetaPropertyTest. A similar approach to the tests for propertyType() might be sensible.

(The links above point to a specific version and should be compared to the current version before starting to work on this issue.)