fix LZW decode error

The dictionary size variable `dict_size` should be 256.
When encode a string like 'aaaa', encoding result is
```
{97, 256, 97}
```
Decoding encounters error using original code. (k = 256, dict_size = 255)

reference
http://rosettacode.org/wiki/LZW_compression#C.2B.2B