Correctly handle \u in Notation 3 files.

It was using case insensitive regex, so it was mistaking \u and \U.
Correctly handle \u in Notation 3 files.

It was using case insensitive regex, so it was mistaking \u and \U.
Complement pull request #426 https://github.com/RDFLib/rdflib/pull/426. Correct failing tests. Add correct case for prefix escape in unicode expansion in litterals
Merge branch 'master' into fix-unicode-escape-n3
Merge remote-tracking branch 'iri-research/master' into fix-unicode-escape-n3

This should have an extra commit that might fix some tests
Merge pull request #426 from rory/fix-unicode-escape-n3

correctly handle \u and \U in n3 files