fix(common): fixed the _.filter clone to not create sparse arrays

Closes #1563

src/common.js

@@ -189,10 +189,12 @@ function pluck(collection, key) {
 }
 
 function filter(collection, callback) {
-  var result = isArray(collection) ? [] : {};
+  var array = isArray(collection);
+  var result = array ? [] : {};
   forEach(collection, function(val, i) {
-    if (callback(val, i))
-      result[i] = val;
+    if (callback(val, i)) {
+      result[array ? result.length : i] = val;
+    }
   });
   return result;
 }

test/commonSpec.js

@@ -0,0 +1,15 @@
+describe('_-like filter', function() {
+  it("should filter arrays", function() {
+    var input = [ 1, 2, 3, 4, 5 ];
+    var filtered = filter(input, function(int) { return int > 2; });
+    expect(filtered.length).toBe(3);
+    expect(filtered).toEqual([ 3, 4, 5 ]);
+  });
+
+  it("should filter objects", function() {
+    var input = { foo: 1, bar: 2, baz: 3, qux: 4 };
+    var filtered = filter(input, function(val, key) { return val > 2; });
+    expect(Object.keys(filtered).length).toBe(2);
+    expect(filtered).toEqual({ baz: 3, qux: 4 });
+  });
+});