Issue2179 incorrect headers

[gitmpje]

Summary of changes

This PR solves issue 2179 "Wrong Content-Type header was sent with query GET request after a POST update request".

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]

Thanks for the PR @gitmpje, copy was not enough, it needed a deepcopy, but the test was great and you found the exact source of the problem which is very helpful.

I changed copy to deepcopy and also changed the test to run everything twice so that any issues with ordering is picked up both ways.

[aucampia]

I will merge by the weekend if there are no further changes.

[coveralls]

Coverage increased (+0.0005%) to 90.633% when pulling 55bcf12 on gitmpje:issue2179_incorrect_headers into 0e61a7f on RDFLib:main.

[gitmpje]

@aucampia Thank you for fixing this issue more fundamentally and resolving the formatting issues. Using copy seemed to solve the issue when testing locally, but deepcopy should definitely fix it.