As part of: issue2551008 I am adding some bs2b calls in the 
roundup/anypy/email_.py in decode_header to make sure that it always 
returns byte strings.

All returns of tuples get a bs2b around the first (header value) 
element.

One place I am not sure about is this code:

     if hasattr(header, '_chunks'):                                             
+        # FIXME do we need bs2b around _charset??                              
         return [(_charset._encode(string, str(charset)), str(charset))         
                     for string, charset in header._chunks]   

do we need a bs2b around the return of _charset? AFAICT there is no
test code that exercises this code.