git-send-email smtpEncryption option is case sensitive (silently fails if "TLS" is specified)

Hi,

I just spend some time figuring out that my newly written git-send-
email configuration didn't work because setting

[sendemail]
    smtpEncryption = TLS

has no effect. This is because the option has to be specified as "tls".
Unfortunately there is no warning that "TLS" is unsupported making it
hard to see why authentication is failing in the end.

I think it would be helpful to have a warning. Additionally one could
also normalize the option to lowercase automatically.

Regards,
Benjamin