fix(voicemail): revent duplicated Content-Type in email

[Stell0]

The problem appears to be caused by the presence of two Content-Type headers in the email message.
Upon examining the email headers, the rule MULTIPLE_UNIQUE_HEADERS was triggered due to the presence of duplicate Content-Type headers:

Content-Type: multipart/mixed; boundary="----voicemail_1350712061221135"
Content-Type: text/plain; charset=utf-8

According to email standards (RFC 2045 and RFC 5322), an email should have only one valid Content-Type header for the entire message.

Since Content-Type, as Date header, is automatically added by s-nail, this PR ensures that only a single Content-Type header is present in the email removing the one that arrives from standard input

[Stell0]

doesn't work. try to set mime-counter-evidence https://manpages.debian.org/stretch/s-nail/s-nail.1.en.html