some improvements to emails.d decoders (after testing against 100.000+ emails)

Hi,

I started to use your characterencoding.d and (parts of) email.d on my current personal project (still in my private repo because is in the prototype stage). After unittesting my code with your modules for my entire collection of emails I wrote these two small changes that made the unittests pass perfectly (parsing body and headers). 

If you want I can repeat the tests using your email object (I'm writing my own email object for my project). 

Thank you for your modules!

Regards,
Juanjo Álvarez