[issue1974] Use space character as continuation_ws

Doc/library/email.header.rst

Index: Doc/library/email.header.rst
===================================================================
--- Doc/library/email.header.rst	(revision 64495)
+++ Doc/library/email.header.rst	(working copy)
@@ -74,7 +74,8 @@
 
    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. The default *continuation_ws* is a single
+   space character (" ").
 
    Optional *errors* is passed straight through to the :meth:`append` method.