Make sure strings are encoded before sending to 'email.header.decode_header'

## Files changed: 5???

But actually, I just moved the unicode stuff to a subdirectory. Besides that, I implemented a 3-ish line change with twice as many lines in comments, in the `decode_email_header` function. This fixes the 11/26 unicode error on PWM. I also made an `ensure_encoded` function that the former function makes use of, which is kind of like the `decode` function on PWM, except it works in reverse.