Clean up concat test;

tests/concat.lua

@@ -30,7 +30,7 @@ describe('concat', function()
     local subject = Rx.Subject.create()
     local onComplete = spy()
     local observable = Rx.Observable.concat(Rx.Observable.fromRange(1, 5), Rx.Observable.fromRange(1, 5), subject)
-    observable:subscribe(function() end, function() end, onComplete)
+    observable:subscribe(nil, nil, onComplete)
     expect(#onComplete).to.equal(0)
     subject:onComplete()
     expect(#onComplete).to.equal(1)