Let jsonld handle value nodes/Literal for context search

[WhiteGobo]

If jsonld parser is set from version 1.0 to 1.1 it looses the ability to handle value nodes that are set as @type for an object. This change should only make the parser ignore value nodes when searching for an already found context. Thus the ability to handle those is not lost, when setting the parser version to 1.1

This behaviour is not expected of the jsonld parser. Other parsers will fail, when a @type is given a literal.
Necessary change to set the default version of the parser to 1.1, because roundtrip test fails otherwise

Background discussion to this change is #2747. (Issue #2606 is the reason for this change.)

Summary of changes

in jsonld-contexts ignore dicts(value nodes), when searching for a context
No changes in tests or documentation, because this is not an expected behaviour.

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.
• 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: 90.999% (-0.009%) from 91.008%
when pulling 7fb9617 on WhiteGobo:jsonfallbackversion11Issue2747
into f792ad5 on RDFLib:main.