add charset encoding to SPARQLConnector.update() request.

[robcast]

Summary of changes

Add encoding "charset=UTF-8" to Content-Type header in SPARQLConnector.update() request.

Fixes #2095

I added a separate test which is basically a copy of the existing test_graph_update(). The assertion could be integrated in the existing test instead.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[aucampia]

Change looks good, I will double check why test is failing when I have time.

[aucampia]

pre-commit.ci autofix

[aucampia]

Thanks for the pull request @robcast - the change and the test looks good.

[coveralls]

Coverage remained the same at 90.634% when pulling 9bccda6 on robcast:issue-2095-set_charset into bcd05e9 on RDFLib:main.