Change default charset when decoding mime parts

[VinceWilkie]

We can safely decode text mime parts with utf-8 instead of ascii because ascii is just a subset of utf-8

[mailgun-ci]

Can one of the admins verify this patch?

[VinceWilkie]

As a side note: decoding text mime parts with ascii can become a problem if it contains unicode information. I have first hand experience with this because I had an email attachment that contained unicode but was decoded using ascii and therefore wasn't properly readable.

[b0d0nne11]

This sounds reasonable but can you add a test for it?