@context: null doesn't work, but @context: [null] does

[cwebber]

The reason? null as falseiness vs null as a value! In fact, api.getContextValue returns null when nothing is there, and the rest of the code uses that as an indicator.

This matters because it means that if we want to support {'@context': null} and not just {'@context': [null]} for context nullification, we're going to need to adjust how that procedure and the code that uses it work.

[davidlehn]

First pass of a hacky fix: https://github.com/digitalbazaar/jsonld.js/pull/289/commits/9c4ac6ce6efd7fc554b14e519324e816e10d1d94

[dlongley]

@davidlehn -- can you see if this is fixed and close if so?