Unexpected exception from EmailAddress.full_spec

[horkhe]

Found in version: 0.6.8

It is possible for EmailAddress.full_spec function to raise an exception even if EmailAddress.require_non_ascii returns False, see a stack trace below:

Traceback (most recent call last):
  File "/app/src/flanker/flanker/mime/message/headers/encoding.py", line 37, in encode
    return encode_unstructured(name, value)
  File "/app/src/flanker/flanker/mime/message/headers/encoding.py", line 52, in encode_unstructured
    return encode_address_header(name, value)
  File "/app/src/flanker/flanker/mime/message/headers/encoding.py", line 65, in encode_address_header
    out.append(addr.full_spec().encode('utf-8'))
  File "/app/src/flanker/flanker/addresslib/address.py", line 536, in full_spec
    ace_hostname = self.hostname.encode('idna')
  File "/usr/lib/python2.7/encodings/idna.py", line 164, in encode
    result.append(ToASCII(label))
  File "/usr/lib/python2.7/encodings/idna.py", line 76, in ToASCII
    label = nameprep(label)
  File "/usr/lib/python2.7/encodings/idna.py", line 38, in nameprep
    raise UnicodeError("Invalid character %r" % c)
UnicodeError: Invalid character u'\u200e'

That breaks the mutual contract of these functions.