Make sure pressure is correctly inferred from MouseEvent button[s] state

Add a test as well
Fixes #120

src/PointerEvent.js

@@ -86,8 +86,10 @@
     // is to call initMouseEvent with a buttonArg value of -1.
     //
     // This is fixed with DOM Level 4's use of buttons
-    var buttons = inDict.buttons;
-    if (buttons === undefined) {
+    var buttons;
+    if (inDict.buttons || HAS_BUTTONS) {
+      buttons = inDict.buttons;
+    } else {
       switch (inDict.which) {
         case 1: buttons = 1; break;
         case 2: buttons = 4; break;

src/dispatcher.js

@@ -57,7 +57,7 @@
     0,
     null,
     // DOM Level 3
-    undefined,
+    0,
     // PointerEvent
     0,
     0,

tests/constructor.js

@@ -6,7 +6,7 @@
 
 suite('Constructor', function() {
   test('PointerEvent extends MouseEvent', function() {
-    var p = new PointerEvent;
+    var p = new PointerEvent();
     expect(p).to.be.an.instanceof(MouseEvent);
   });
 
@@ -63,12 +63,27 @@ suite('Constructor', function() {
       'hwTimestamp',
       'isPrimary'
     ];
-    var p = new PointerEvent;
+    var p = new PointerEvent();
     var v;
     props.forEach(function(k) {
       v = p[k];
       p[k] = NaN;
       expect(p[k]).to.equal(v);
     });
   });
+
+  test('Button properties are used for pressure', function() {
+    var p = new PointerEvent('foo', {buttons: 1});
+    expect(p.pressure).to.equal(0.5);
+    // test for buttons property
+    var m = document.createEvent('MouseEvent');
+    m.initEvent('test', false, false, null, null, 0, 0, 0, 0, false, false, false, false, 0, null);
+    // only run this test if <MouseEvent>.buttons is not supported
+    if (m.buttons === undefined) {
+      p = new PointerEvent('baz', {button: 0, which: 1});
+      expect(p.pressure).to.equal(0.5);
+    }
+    p = new PointerEvent('bar');
+    expect(p.pressure).to.equal(0);
+  });
 });