refactor filters and corresponding tests

src/util/underscore.js

@@ -1,5 +1,29 @@
-function isString(value){
+/*
+ * Checks if value is classified as a String primitive or object.
+ * @param {any} value The value to check.
+ * @return {Boolean} Returns true if value is a string, else false.
+ */
+function isString(value) {
     return value instanceof String || typeof value === 'string';
 }
 
+/*
+ * Iterates over own enumerable string keyed properties of an object and invokes iteratee for each property. 
+ * The iteratee is invoked with three arguments: (value, key, object). 
+ * Iteratee functions may exit iteration early by explicitly returning false.
+ * @param {Object} object The object to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @return {Object} Returs object.
+ */
+function forOwn(object, iteratee) {
+    object = object || {};
+    for (var k in object) {
+        if (object.hasOwnProperty(k)) {
+            if (iteratee(object[k], k, object) === false) break;
+        }
+    }
+    return object;
+}
+
 exports.isString = isString;
+exports.forOwn = forOwn;