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 problem is that hmac does not deal with unicode like the error messages makes the user believe, and throws this error instead.

I should note that this is not the best solution, because dropping unicode characters is not a good idea when you are dealing with a secret key, however, it seems to be the only solution i could think of, and is already implemented for the string (2 lines above my changes) object.

It's been pointed out that a Secret Key should be a random bytestring, and not unicode. I understand this, however feel that it is good practice to be safe about this.