"Port" the fix for issue 1974 from the trunk (2.7). Because Python 3.x does things much better, less changes are necessary.
This also shoves test_email_codecs_rename.py onto test_email_codecs.py even though the test needs to be ported to Python 3.
diff --git a/Doc/library/email.header.rst b/Doc/library/email.header.rst
index 5f37264..7ee4a2d 100644
--- a/Doc/library/email.header.rst
+++ b/Doc/library/email.header.rst
@@ -72,7 +72,7 @@
Optional *continuation_ws* must be :rfc:`2822`\ -compliant folding whitespace,
and is usually either a space or a hard tab character. This character will be
- prepended to continuation lines.
+ prepended to continuation lines. *continuation_ws* defaults to a single space character (" ").
Optional *errors* is passed straight through to the :meth:`append` method.