Default sender address incompatible with mime_header_encode() function

The mimemail_address() function return an array with name and address, if no sender address have been provided.

This array is passed to mime_header_encode() function a few lines later in mimemail_prepare_message(). mime_header_function() requires a string as it test the address in a preg_match.

The default address should be passed under its string form ( name
) instead of in an array.