Add Observable.switch test;

tests/observable.lua

@@ -267,6 +267,7 @@ describe('Observable', function()
   dofile('tests/skipLast.lua')
   dofile('tests/startWith.lua')
   dofile('tests/sum.lua')
+  dofile('tests/switch.lua')
   dofile('tests/take.lua')
   dofile('tests/takeUntil.lua')
   dofile('tests/takeWhile.lua')