fixing securecookie.py

was having a problem with the hmac module in python dealing with unicode strings in this module, so i checked the instance and dealt with it if it happened like this.

basically hmac does not deal with unicode at all, and it creates a problem when inputting a unicode secret key, namely the following:
```python
TypeError: character mapping must return integer None or unicode
```
the probem is that hmac does not deal with unicode like the error messages makes the user believe, and throws this error instead.