fix namespace prefixes in longturtle serialization

[ddeschepper]

Summary of changes

Solves #3105 by storing the namespace manager of the graph that is to be serialized temporarily and reapplying it to the graph that is actually serialized by the implementation of the longturtle serializer.

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.

[edmondchuc]

Thank you for this! I had a local PR that I wrote up on the weekend but you beat me to it. :)

This partially resolves #3062 but just noting here the prefix declaration for rdf is still not preserved when the value rdf:type is used in the object position of a triple.

[ddeschepper]

@edmondchuc is there anything I can do to help this get merged? I see some failing checks, but I'm unsure what causes them.