fix(symbol): treat null value the same as undefined

Fixes #13

src/symbol.js

@@ -248,7 +248,7 @@ import {PLATFORM} from 'aurelia-pal';
           descriptor.value = function () {
             var
               str = toString.call(this),
-              tst = typeof this === 'undefined' ? undefined : this[Symbol.toStringTag]
+              tst = typeof this === 'undefined' || this === null ? undefined : this[Symbol.toStringTag]
             ;
             return typeof tst === 'undefined' ? str : ('[object ' + tst + ']');
           };