allow strictBytes type for secret

[hampsterx]

we have a secret key that is in bytes (not utf-8 unfortunately).

PyJWT does this..

def force_bytes(value):
    if isinstance(value, text_type):
        return value.encode('utf-8')
    elif isinstance(value, binary_type):
        return value
    else:
        raise TypeError('Expected a string value')

eg it allows to use bytes for the key.

Modifying type confirms works fine.