Test to verify that toRDF() on array contexts fail
Test with term2, not ex:term2 (to get @type:@id)
Don't assume @context are Map - also handle List

TODO: Also handle String (e.g. external)
Add getPrefixes() to expose term definitions to RDFDataset
Use Context to parse contexts and find prefixes
assert returned namespaces
Only 'common' prefixes returned - not every term
Ignore purl.org test as purl.org is unstable (again)
Reformat to use our style
Merge branch 'master' into array-context