Fix bug when asserting some valid ES6 keys

- Resolution of chaijs/chai#674
- Add compareByInspect utility for use with assertKeys sorts
- Add getOwnEnumerableProperties utility
- Add getOwnEnumerablePropertySymbols utility
- Add Symbol support to the inspect utility
- Add tests to utilities, should, expect, and assert

lib/chai/core/assertions.js

@@ -1170,7 +1170,7 @@ module.exports = function (chai, _) {
       }
 
     } else {
-      actual = Object.keys(obj);
+      actual = _.getOwnEnumerableProperties(obj);
 
       switch (_.type(keys)) {
         case 'array':
@@ -1184,7 +1184,10 @@ module.exports = function (chai, _) {
           keys = Array.prototype.slice.call(arguments);
       }
 
-      keys = keys.map(String);
+      // Only stringify non-Symbols because Symbols would become "Symbol()"
+      keys = keys.map(function (val) {
+        return typeof val === 'symbol' ? val : String(val);
+      });
     }
 
     if (!keys.length) throw new Error('keys required');
@@ -1248,8 +1251,8 @@ module.exports = function (chai, _) {
         ok
       , 'expected #{this} to ' + str
       , 'expected #{this} to not ' + str
-      , expected.slice(0).sort()
-      , actual.sort()
+      , expected.slice(0).sort(_.compareByInspect)
+      , actual.sort(_.compareByInspect)
       , true
     );
   }