Add test for adding JSON-LD to guess_format()

This is a follow-on patch to:
e778e94

Test was confirmed to pass by running this on the current `master`
branch, and confirmed to fail with e778e94 reverted.

    nosetests test/test_parse_file_guess_format.py

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

test/test_parse_file_guess_format.py

@@ -10,6 +10,10 @@
 
 
 class FileParserGuessFormatTest(unittest.TestCase):
+    def test_jsonld(self):
+        g = Graph()
+        self.assertIsInstance(g.parse("test/jsonld/1.1/manifest.jsonld"), Graph)
+
     def test_ttl(self):
         g = Graph()
         self.assertIsInstance(g.parse("test/w3c/turtle/IRI_subject.ttl"), Graph)