Refactor annotation rectangle code and add unit tests

This patch refactors the code responsible for setting the annotation's rectangle. Its goal is to:

- Actually check that the input array is actually an array, and if so, that it contains exactly four elements.
- Only call `normalizeRect` if the input array is valid, i.e., we do not call it for the default rectangle anymore.

Unit tests are provided just like with the other patches in this series.

src/core/annotation.js

@@ -75,10 +75,11 @@ var Annotation = (function AnnotationClosure() {
     var data = this.data = {};
 
     data.subtype = dict.get('Subtype').name;
-    var rect = dict.get('Rect') || [0, 0, 0, 0];
-    data.rect = Util.normalizeRect(rect);
     data.annotationFlags = dict.get('F');
 
+    this.setRectangle(dict.get('Rect'));
+    data.rect = this.rectangle;
+
     this.setColor(dict.get('C'));
     data.color = this.color;
 
@@ -91,6 +92,21 @@ var Annotation = (function AnnotationClosure() {
   }
 
   Annotation.prototype = {
+    /**
+     * Set the rectangle.
+     *
+     * @public
+     * @memberof Annotation
+     * @param {Array} rectangle - The rectangle array with exactly four entries
+     */
+    setRectangle: function Annotation_setRectangle(rectangle) {
+      if (isArray(rectangle) && rectangle.length === 4) {
+        this.rectangle = Util.normalizeRect(rectangle);
+      } else {
+        this.rectangle = [0, 0, 0, 0];
+      }
+    },
+
     /**
      * Set the color and take care of color space conversion.
      *