Avoid exception on @context with default @language and unmapped key

Fix NullPointerException occurring when compacting a document using
a @context with a default @language that does not explicitly map a
key that appears in the document (see included test case).
Update test manifest to conform to upstream test suite

See https://github.com/json-ld/json-ld.org/pull/335
Merge branch 'master' into array-context