Remove charset param from multipart content-types

As described in http://dev.rubyonrails.org/ticke..., the AOL e-mail client incorrectly displays all parts of a multipart/alternative section when a charset parameter is listed. The patch submitted at the time fixed this for the message's overall content-type. However, when sending a multipart/mixed message with a multipart/alternative section, the Content-Type header for the alternative section still includes charset parameter and AOL still renders both parts.

This patch removes the charset parameter from that header, which causes AOL's desktop client to properly display the message.