Add a break to avoid case fallthrough

In JavaScript, when there isn't a `break` or `return` statement inside a
`switch` case, the next case will be executed as well.

That wasn't a problem here because CoffeeScript automatically inserts a
`return` statement in this specific situation. However, since we don't
want to return the result of the `hide()` operation, it might be easy to
accidentally remove the `return` statement, causing the code to break.

I've added a test for the scenario where neither `break` nor `return`
statements are present, so we don't run into this error.

app/assets/javascripts/polls_admin.js.coffee

@@ -7,6 +7,7 @@ App.PollsAdmin =
           when "vote_collection"
             $("select[class='js-shift-vote-collection-dates']").show()
             $("select[class='js-shift-recount-scrutiny-dates']").hide()
+            break
           when "recount_scrutiny"
             $("select[class='js-shift-recount-scrutiny-dates']").show()
             $("select[class='js-shift-vote-collection-dates']").hide()