class Mail::Message

def add_mime_version(ver_val = '')

It will preserve any date you specify if you do.

specifying a version yourself.
set itself to '1.0' if you try and encode it or output it to_s without
into the Header. The MimeVersion object will automatically generate
Creates a new empty Mime Version field and inserts it in the correct order
def add_mime_version(ver_val = '')
  header['mime-version'] = ver_val
end