Prevent "DZ" from being decoded to an unicode character.

Thanks for the wonderful and simple-to-use package! I use it for "cleaning" .bib files when converting it to other bibliographic formats. The only problem I had was that "DZ" that was sometimes used as the abbreviations of the first and middle name was converted in an Unicode character. This small patch solved my problem: Now DZ is treated like other double ASCII characters like e.g. NJ where the parameter "decode" is False as well:

```python
self.register(u'\N{LATIN CAPITAL LETTER NJ}', b'NJ', decode=False)
```