Issue with namespace rebinding

[aucampia]

This was reported by @gjhiggins and a test to reproduce it was added here:

https://github.com/aucampia/rdflib/blob/32341bb28e0d2a4b828519cd452959e177a3c219/test/test_namespace_rebinding.py#L16-L205

I'm not going to explain the details of the issue, just making this to track the bug as it should be fixed.

A previous attempt to fix this was made in #1800 - however this PR has some deficiencies and needs to be redone on master.