feat: change dataset's default serialize format to trig

[edmondchuc]

Summary of changes

Previously, calls to a dataset's serialize method used the superclass (Graph)'s default format, "turtle". However, serializing a dataset with the turtle format results in only triples in the default graph, with named graph triples omitted. A better default format for datasets is "trig", which serializes all triples from all graphs.

This PR changes the default serializer format for datasets to "trig".

This was raised in this issue: #2021

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.