Add tests for RDF serialization when oslc.properties selects nested resource properties.

[berezovskyi]

The serializer uses onlyNested to distinguish between:

• properties explicitly selected for output;
• properties encountered only because a nested wildcard is being applied.

Scalar values encountered only for nested traversal should be omitted, while resource-valued properties should still be serialized so their selected nested properties can be emitted.

OSLC4Net has not exercised this behavior extensivelyin the past, we should increase test coverage.

Scenarios

• Unfiltered serialization includes all populated scalar and resource properties.
• Explicit selection of a scalar date property includes that property.
• A nested wildcard selection omits unrelated top-level scalar date properties (inspired by core: fix DateTimeOffset serialization and deserialization #857 ).
• A nested wildcard still emits the selected resource relationship and its nested properties.
• A combined top-level wildcard and nested wildcard includes top-level scalar properties while filtering nested resource contents.
• The behavior is consistent across RDF/XML and Turtle serialization.