prevent any future encoding errors

I'd prefer broken emails over getting no emails ...

@jeremy @bf4 @mikel 


Fix:

```Ruby
# https://github.com/mikel/mail/pull/768
# prevent any kind of encoding foobar
module PreventEncodingExceptions
  def value_decode(str)
    super
  rescue ArgumentError, Encoding::ConverterNotFoundError
    warn "Invalid encoding caught #{$!}"
    str
  end
end
prepend PreventEncodingExceptions
```
I prefer using the constant form Encoding::UTF_8 for load-time checking of the desired encoding so as to avoid typos... but I suppose the file is already using the string form all over.. sigh