Instead of this loop and join, which are arguably inefficient, I think we can change this to:
```
  return b.toString('utf16-be');
```

This is what I did in my decodeModifitedUtf7 routine in our forked imap.js.

I grok we're taking existing code, but since we're doing this for Outlook and we know its HTML output can get verbose, I think it's worth optimizing a little.