mb_convert_encoding breaks code about imap

[sergbass]

Also, mb_convert_encoding breaks when unsupported encoding occurs, for instance, windows-1257. And there's no way to avoid code break. I suggest to do 2 things:

1. add workarounds for known unsupported encodings, may be for this purpose separate class could be created;
2. add fallback for other unexpected unsupported encodings: when source encoding isn't in mb_list_encodings, set it to auto.

[spshe4]

@ddeboer
I got this error "mb_convert_encoding(): Unable to detect character encoding".
I use - "ddeboer/imap": "^0.5.2"

Can I fix something in my code so that I do not change the vendor files?