Fix btoa to match browser implementations

Test case:
btoa(String.fromCharCode(252)) should return "/A=="

index.js

@@ -2,7 +2,7 @@
   "use strict";
 
   function btoa(str) {
-    return new Buffer(str, 'utf8').toString('base64');
+    return new Buffer(str, 'binary').toString('base64');
   }
 
   module.exports = btoa;