Remove charset from multipart content-types

Had an issue with ActionMailer when trying to send messages to AOL users. Looks like someone ran into this a long time ago (http:// dev.rubyonrails.org/ticket/2013). However, I was trying to send a multipart/mixed message, with a multipart/alternative section and an additional attachment. The alternative section was still being assigned a default charset, causing AOL to render the message improperly. This patch removes that charset param and adds a line to the nested_multipart test to verify.

http://rails.lighthouseapp.com/projects/8994/tickets/2109-remove-charset-param-from-multipart-content-types

Thanks, Elliott