Check that by default sparql accepts xml

[WhiteGobo]

See #3332 for more discussion.
Prevent, that by chance the default behaviour of sparqlstore, to accept xml on a query is changed.
This change may also be incorporated into PR #3324

Summary of changes

Added check for xml at test test.test_store.test_store_sparqlstore_query.test_query_construct_format

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.

[coveralls]

coverage: 90.331%. first build
when pulling af5c216 on WhiteGobo:patch-2
into 72a98ed on RDFLib:main.