string containing \x00 raises unexpected exception.

string containing \x00 is being mishandled by `strlen()`, because `strlen()` only counts from start to  NULL(\x00).

On the current version, I could get these error messages:
    
    >>> blake2.blake2('\x00')
    >>> 
    TypeError: argument 1 must be string without null bytes or None, not str

Thus I changed some codes, please check this out.

p.s. I also added some test codes for your convenience. ^^