Support JSON array in @context with toRDF

[stain]

JsonLdProcessor.toRDF failed if options.useNamespaces = true
and the input has an @context which is an array rather than an object - the code tried to cast to Map.

Failing JSON-LD (as verified by included test ArrayContextToRDFTest and src/test/resources/custom/array-context.jsonld):

{
  "@context": [
    "http://nonexisting.example.com/context",
    { "ex2": "http://example.com/2/" }
  ],
  "@id": "ex2:a",
  "term2": "ex2:b"
}

This suggested fix is to reuse Context to recursively parse the contexts (including doing any imports) and remove the casting.

Additionally this fix ensures that only 'common' prefixes (e.g. where the URI end with / or #) are returned - thus not poluting the RDF namespaces with "term1", "term2" etc. -- if controversial this can be reversed in the call to context.getPrefixes(true) by changing to false.

[ansell]

I will look into testing and merging this. Are you using Eclipse by the way? If so, you can install the code formatting and code cleanup configurations from the /conf/ directory to ensure that the code in pull requests is cleanly formatted and only shows the relevant changes. I am not too worried about tabs versus spaces, but we chose 4 spaces, so the current files in Git are formatted that way.

I will pull into a local branch and reformat the files for this pull request to match our styles.