Recent changes cause a few tests/test_handlers.py tests to fail

[Mark Sapiro]

These failures occur in 2.1.17:

======================================================================
FAIL: test_list_headers (test_handlers.TestCookHeaders)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/MM/21/tests/test_handlers.py", line 707, in test_list_headers
    eq(msg['list-archive'], '<http://www.dom.ain/pipermail/_xtest>')
AssertionError: '<http://www.dom.ain/pipermail/_xtest/>' != '<http://www.dom.ain/pipermail/_xtest>'

======================================================================
FAIL: test_convert_to_plaintext (test_handlers.TestMimeDel)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/MM/21/tests/test_handlers.py", line 1224, in test_convert_to_plaintext
    eq(msg.get_payload(), '\n\n\n')
AssertionError: '' != '\n\n\n'

======================================================================
FAIL: test_send_a_digest (test_handlers.TestToDigest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/MM/21/tests/test_handlers.py", line 1886, in test_send_a_digest
    eq(len(files), 2)
AssertionError: 0 != 2