itsdangerous throws BadSignatureException when using URLSafeSerializer with '_' separator

[tjerkw]

This code fails:

def test(count=1030):
    import itsdangerous
    secret = 'test'
    s = itsdangerous.URLSafeSerializer(secret, signer_kwargs={'sep': '_'})
    for i in xrange(count):
        val = gen_random_string(len=3)
        s.loads(s.dumps(val))

Improvment: let itsdangerous throw an exception if an invalid seperator is used.

[tjerkw]

I also thought the string has BASE64 + sep + BASE64, but apparently that's not the case.

[davidism]

I can't reproduce this issue. _ (and any character not used by base64) is a valid separator. I think it's reasonable to assume the user isn't picking a character that will be used by the encoding.