This commit fixes issues with the 16501 module for Mojolicious cookies.
- Fix parsing of message: The base64 encoded message has '=' characters
replaced with '-' meaning that three such characters can be encountered
at the end of the message. This commit sets a fixed size for the
message and doesnt use a `sep` to parse it.
- Update self-test with real-world mojolicious cookie.
- Update the test module to reflect what's actually happening when
Mojolicious generates a signed cookie.