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"
     },