url_safe_base64 replacement_chars arrays

Think you've got them the wrong way around.

Try this (equivalent to what happens when the plugin encodes):

```
$replacement_chars = array(
 '.' => '+',
 '-' => '=',
 '~' => '/'
);
echo strtr("split/string",$replacement_chars);
```

The / is still there in the output.

Think you you need the other set from the decode function in encode and v.v.