Fix spec for window scroll

The spec that I previously wrote for when the scrollable parent is equal
to the window element (a3dc459) only worked in Chrome. Our Travis
integration runs a Firefox instance, and in that browser the spec
failed. It turns out that Chrome will allow you to do `window.scrollTop
= x` and it will scroll to x. Firefox silently ignores that call. The
way we found to work in both browsers were `window.scroll(0, x)`.

Once that was fixed, I found another issue with our test setup. We
weren't cleaning up the divs we leave on the page. This caused the last
spec (testing scroll on the window element) to be contaminated by those
divs. We added an afterEach hook to unmount components as they were
tested.

spec/waypoint_spec.js

@@ -1,15 +1,21 @@
 var React = require('react');
 var Waypoint = require('../src/waypoint');
 
+var div;
+
 var renderAttached = function(component) {
-  var div = document.createElement('div');
+  div = document.createElement('div');
   document.documentElement.appendChild(div);
   var renderedComponent = React.render(component, div);
   return renderedComponent;
 };
 
 var scrollNodeTo = function(node, scrollTop) {
-  node.scrollTop = scrollTop;
+  if (node === window) {
+    window.scroll(0, scrollTop);
+  } else {
+    node.scrollTop = scrollTop;
+  }
   var event = document.createEvent('Event');
   event.initEvent('scroll', false, false);
   node.dispatchEvent(event);
@@ -44,6 +50,12 @@ describe('Waypoint', function() {
     };
   });
 
+  afterEach(function() {
+    if (div) {
+      React.unmountComponentAtNode(div);
+    }
+  });
+
   describe('when the Waypoint is visible on mount', function() {
     beforeEach(function() {
       this.topSpacerHeight = 90;
@@ -245,8 +257,8 @@ describe('Waypoint', function() {
       this.parentStyle = {};
 
       // Make the spacers large enough to make the Waypoint render off-screen
-      this.topSpacerHeight = 4000;
-      this.bottomSpacerHeight = 4000;
+      this.topSpacerHeight = window.innerHeight + 1000;
+      this.bottomSpacerHeight = 1000;
     });
 
     it('does not fire the onEnter handler on mount', function() {
@@ -256,7 +268,7 @@ describe('Waypoint', function() {
     describe('when the Waypoint is in view', function() {
       beforeEach(function() {
         this.subject();
-        scrollNodeTo(window, 3900);
+        scrollNodeTo(window, this.topSpacerHeight - window.innerHeight / 2);
       });
 
       it('fires the onEnter handler', function() {