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
```
https://github.com/mikel/mail/pull/785 looking good ?


On Tue, Aug 26, 2014 at 7:23 PM, Michael Grosser <grosser.michael@gmail.com>
wrote:

> 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>.
>>
>
>