Blank node as predicate tests

From https://github.com/digitalbazaar/jsonld.js/issues/498.

cc/ @pchampin @davidlehn

(the same issue is present in PyLD: https://github.com/digitalbazaar/pyld/issues/167)

Consider the following:

{
  "@context": {
    "t": "_:b"
  },
  "@type": "t:x"
}

It expands to

[
  {
    "@type": [
      "t:x"
    ]
  }
]

while it should expand to

[
  {
    "@type": [
      "_:bx"
    ]
  }
]

Indeed, step 14.2.5 of Create Term Definition clearly states:

If term contains neither a colon (:) nor a slash (/), simple term is true, and if the IRI mapping of definition is either an IRI ending with a gen-delim character, or a blank node identifier, set the prefix flag in definition to true.

(emphasis is mine)

Therefore, step 6.4 of IRI Expansion should apply.

Start with the JSON-LD example in the issue and read the linked Create Term Definition and IRI Expansion algorithms, especially steps 14.2.5 and 6.4. Compare the current behavior with the expected expansion shown here; the work is done when blank node identifiers are handled according to those steps and the behavior is covered by the relevant tests.