force undefined contexts passed to test be false rather than throwing…

… errors. Fixes #4

lib/rule.js

@@ -35,6 +35,7 @@ function Rule(path, ruler){
  */
 
 Rule.prototype.test = function(obj){
+  if ('undefined' == typeof obj) return false;
   var actual = this.path(obj);
   return this.stack.every(function(fn){
     return fn(actual);

test/rule.test.js

@@ -93,4 +93,12 @@ describe('Rule', function(){
     });
   });
 
+  describe('undefined contexts', function(){
+    it('should not throw an exception', function(){
+      var rule = new Rule('name');
+      rule.eq('hai');
+      rule.test().should.equal(false);
+    });
+  });
+
 });