ability to select the document node with $(document)

enables usage like `$(document).delegate(...)`

src/zepto.js

@@ -30,14 +30,15 @@ var Zepto = (function() {
   }
 
   function $(selector, context){
-    if (selector == document) return Z();
-    else if (context !== undefined) return $(context).find(selector);
+    if (!selector) return Z();
+    if (context !== undefined) return $(context).find(selector);
     else if (typeof selector === 'function') return $(document).ready(selector);
     else if (selector instanceof Z) return selector;
     else {
       var dom;
       if (selector instanceof Array) dom = compact(selector);
-      else if (selector instanceof Element || selector === window) dom = [selector];
+      else if (selector instanceof Element || selector === window || selector === document)
+        dom = [selector], selector = null;
       else if (fragmentRE.test(selector)) dom = fragment(selector);
       else dom = $$(document, selector);
       return Z(dom, selector);
@@ -106,7 +107,7 @@ var Zepto = (function() {
       var node = this[0], nodes = $$(context !== undefined ? context : document, selector);
       if (nodes.length === 0) node = null;
       while(node && node !== document && nodes.indexOf(node) < 0) node = node.parentNode;
-      return $(node);
+      return $(node !== document && node);
     },
     parents: function(selector){
       var ancestors = [], nodes = this;

test/zepto.html

@@ -203,6 +203,13 @@ <h1>Zepto DOM unit tests</h1>
         t.assertLength(1, $('p > span.yay'));
       },
 
+      testDollarWithNil: function(t){
+        t.assertLength(0, $(null));
+        t.assertLength(0, $(undefined));
+        t.assertLength(0, $(false));
+        t.assertLength(0, $(''));
+      },
+
       testSize: function(t){
         t.assertEqual(4, $('#find1 .findme').size());
       },
@@ -244,7 +251,7 @@ <h1>Zepto DOM unit tests</h1>
 
       testDollarWithDocument: function(t){
         var z = $(document);
-        t.assertLength(0, z);
+        t.assertLength(1, z);
         t.assertEqual('', z.selector);
       },