Fixes for #467

Decode string in Content-disposition header parameter such as "filename" properly.

Do we want to change this behavior? here and in spec/mail/fields/content_type_field_spec.rb It looks like a regression to me https://github.com/mikel/mail/commit/4d9b61a291fa2cbcd691c01b596614d25065ea8e

instead of `str.encode(Encoding::UTF_8, :invalid => :replace, :replace => "")` replacing with `''` maybe we should use the default  [`U+FFFD �`](http://graphemica.com/%EF%BF%BD)?