fix multipart post for servers that choke on Content-Type

Some servers such as Exchange 2010 don't like that there isn't a space
after a semicolon separator in the Content-Type request header. Since
Net::HTTPHeader#set_content_type adds the space, as well as it appearing
in the examples of RFC 2616, this follows suit and adds a space which
should help bad-behaving servers and not harm others.