py3: work around weird handling of bytes/unicode in decode_header()

Basically decode_header() works as follows, and on Python 3, email headers
ARE UNICODE.

  def decode_header(header):
      if not ecre.search(header):  # ecre is unicode regexp
          return [(header, None)]  # so header is unicode string
      ... decode header into [(bytes_data, unicode_charset_name)]
      return collapsed

mercurial/mail.py

@@ -332,6 +332,11 b' def headdecode(s):'
                 continue
             except UnicodeDecodeError:
                 pass
+        # On Python 3, decode_header() may return either bytes or unicode
+        # depending on whether the header has =?<charset>? or not
+        if isinstance(part, type(u'')):
+            uparts.append(part)
+            continue
         try:
             uparts.append(part.decode('UTF-8'))
             continue