Set plugin list as a @set and fixed test case

It turns out setting "plugins" as a @list in the context causes the
"plugins" property to expand to its full name.
Removing the type causes a regression of #17, which I initially missed
because the test in #17 was wrong.

Closes #26

senpy/schemas/context.jsonld

@@ -40,6 +40,9 @@
     "options": {
       "@container": "@set"
     },
+    "plugins": {
+      "@container": "@set"
+    },
     "prov:wasGeneratedBy": {
       "@type": "@id"
     },

tests/test_models.py

@@ -170,11 +170,10 @@ class ModelsTest(TestCase):
     def test_single_plugin(self):
         """A response with a single plugin should still return a list"""
         plugs = Plugins()
-        for i in range(10):
-            p = Plugin({'id': str(i),
-                        'version': 0,
-                        'description': 'dummy'})
-            plugs.plugins.append(p)
+        p = Plugin({'id': str(0),
+                    'version': 0,
+                    'description': 'dummy'})
+        plugs.plugins.append(p)
         assert isinstance(plugs.plugins, list)
         js = plugs.jsonld()
         assert isinstance(js['plugins'], list)