Modifies EncryptionLevel to Off instead of NotSupported

[boersmamarcel]

Resolves #707

[boersmamarcel]

@pangjunrong could you help out with this merge? If there are additional steps I need to take then please let me know. Or could you refer me to somebody else to help get this merge approved?

[pangjunrong]

So I spent some time understanding the underlying mechanisms for the different encryption options from [MS-SSTDS]: Tabular Data Stream Protocol Version 4.2 (specifically 2.2.6.4) and came across several possible issues with using EncryptionLevel::Off, such as tiberius #320.

I think the most comprehensive way forward would be to account for all possible DB setups — in your case, the expected connection might not be encrypted but EncryptionLevel::Off is necessary because a TLS handshake is still expected during PRELOGIN. For someone else, their setup might not support TLS at all and having EncryptionLevel::Off might lead to a timeout as the client attempts a secured authentication handshake prior to the unencrypted connection.

May I propose that when encrypt=false, we do EncryptionLevel::Off and otherwise we default to EncryptionLevel::NotSupported?

For the merge to be approved, you will need @wangxiaoying

[wangxiaoying]

Thanks @boersmamarcel for the PR!

I agree with @pangjunrong that we might want to have both EncryptionLevel::Off and EncryptionLevel::NotSupported supported.