let me set a default charset

I'd prefer iso-8859-1, let me set it without doing monkeypatches

@jeremy @bf4

atm I'm using this:
```Ruby
module DefaultCharset
  def init_with_string(*args, &block)
    @charset = 'iso-8859-1'
    super
  end
end

Mail::Message.prepend(DefaultCharset)
```
Wouldn't use setting a constant as API. Use a writer method.