support windows-1258

fixes https://github.com/mikel/mail/issues/544

Fix:
```Ruby
module Windows1258
  def pick_encoding(enc)
    case enc
    when /windows-?1258/i
      'Windows-1252'
    else
      super
    end
  end
end
class << Mail::Ruby19
  prepend Windows1258
end
```
I think a hook to do your own conversion logic would be very helpful ...
would kill a bunch of my monkey patches ... I'll try a PR ...


On Tue, Aug 26, 2014 at 6:45 PM, Benjamin Fleischer <
notifications@github.com> wrote:

> In spec/mail/field_spec.rb:
>
> > @@ -260,6 +260,11 @@
> >        expect(subject.decoded).to eq "It’s a test?"
> >      end
> >
> > +    it "should decode Windows-1258" do
> > +      subject = Mail::SubjectField.new("=?windows-1258?Q?C=E9drine?=", 'utf-8')
> > +      expect(subject.decoded).to eq "Cédrine"
>
> Perhaps we could support an easier hook for end-users to add their own
> mappings in the meantime?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/mikel/mail/pull/767/files#r16753862>.
>