Fix the issue with local name during the event creation with user extensions.

[Aravinda93]

Currently, during the addition of user extensions, the default local names such as GS1, CBV, etc are considered in @Context, as it is part of the epics-context schema it should not be considered in context.

Also, some of the user extension local names are not considered correctly such as:

http://epcis.example.org/myvoc results in the local name of epcis whereas the actual local name should be myvoc.

[Aravinda93]

A similar issue has been noted in the new version:

Current context consists of following:

{ "ref": "https://www.gs1.org/voc/" }

expected: { "gs1": "https://www.gs1.org/voc/" }