FIX: Show every voter only once. (#6746)

Each voter is represented by an object and Set does not properly check for equality.

plugins/poll/test/javascripts/acceptance/polls-test.js.es6

@@ -1344,7 +1344,7 @@ test("Public poll", async assert => {
 
   assert.equal(
     find(".poll-voters:first li").length,
-    50,
+    26,
     "it should display the right number of voters"
   );
 });