sparqls optionals clause can now bind variables. with test. issue 2957

[WhiteGobo]

Summary of changes

added test for issue #2957
working patch to fix issue

in technical terms sparql plugin now merges the results of optional with previous results.

also fix mentioned in pr #3077

fix should be backwards compatible

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 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.

[coveralls]

coverage: 90.299%. remained the same
when pulling 756d933 on WhiteGobo:issue2957subselectoptional
into 34751ac on RDFLib:main.