Correct and fast RC4 stream cipher for JavaScript

benchmark (see test.html):

Chrome 37:   185.98884066955983MB/s
Safari 7:    55.0357732526142MB/s
Firefox 32:  234.55824863174354MB/s

usage:

var key = new Uint8Array(16);
var plainText1 = new Uint8Array(10);
var plainText2 = new Uint8Array(17);

var cipherText1 = new Uint8Array(10);
var cipherText2 = new Uint8Array(17);

var cipher = new RC4(key);
cipher.update(cipherText1, plainText1, 10);
cipher.update(cipherText2, plainText2, 17);
// ...