Changing btoa ponyfill to improve testing

lib/helpers/btoa.js

@@ -4,12 +4,12 @@
 
 var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
 
-function InvalidCharacterError(message) {
-  this.message = message;
+function E() {
+  this.message = 'String contains an invalid character';
 }
-InvalidCharacterError.prototype = new Error;
-InvalidCharacterError.prototype.code = 5;
-InvalidCharacterError.prototype.name = 'InvalidCharacterError';
+E.prototype = new Error;
+E.prototype.code = 5;
+E.prototype.name = 'InvalidCharacterError';
 
 function btoa(input) {
   var str = String(input);
@@ -26,7 +26,7 @@ function btoa(input) {
   ) {
     charCode = str.charCodeAt(idx += 3 / 4);
     if (charCode > 0xFF) {
-      throw new InvalidCharacterError('INVALID_CHARACTER_ERR: DOM Exception 5');
+      throw new E();
     }
     block = block << 8 | charCode;
   }