Load JSON-LD contexts from URL using input stream method of JSONFactory

Avoids problems with content type which breaks URL.getContent() - 
instead we make a URLConnection with the appropriate Accept headers. 

Note that this assumes UTF-8 encoding which is required by JSON
specification of application/json (and thus for application/ld+json)
Tidy..
Test request property using fake URLStreamHandler
Swapped order so application/ld+json is first