Handle case for when object to serialize is of bytes type, fixes #380

[RaheelaKhan1172]

This fixes the bug in #380. The issue was a bytes object not being handled in _CustomJsonEncoder. You can also fix this here:

sslyze/sslyze/utils/tls_wrapped_protocol_helpers.py

Line 257 in c77ed63

raise StartTlsError(self.ERR_NO_STARTTLS)

by explicitly passing a string by decoding self.ERR_NO_STARTTLS. I'm not sure which method you'd prefer.
I can add a test too if it's helpful - let me know what you think!

I forgot to post the command I used to reproduce the bug:
sslyze/__main__.py --tlsv1_2 --json_out=- --starttls=imap imap-mail.outlook.com:110

and the result I get after the update:

{
    "accepted_targets": [],
    "invalid_targets": [
        {
            "imap-mail.outlook.com:110": "IMAP START TLS was rejected"
        }
    ],
    "sslyze_url": "https://github.com/nabla-c0d3/sslyze",
    "sslyze_version": "2.1.3",
    "total_scan_time": "0.06627202033996582"
}

[nabla-c0d3]

I ended up fixing it the other way ( e97f20f ). Thanks for looking into it!