fixed writeBytes default arguments, cleaned up readBytes'

* `writeBytes` was clearly not working like in AS3, as passing only one argument (`bytes`) would result in writing zero bytes (instead of `bytes.length`)
* I also just slightly changed the way both `readBytes` and `writeBytes` initializes default values, to make them more symetric to each other, and closer to AS3's documentation

From my tests, it works much better than before :)