[issue1974] Use space character as continuation_ws

Lib/email/generator.py

Index: Lib/email/generator.py
===================================================================
--- Lib/email/generator.py	(revision 64495)
+++ Lib/email/generator.py	(working copy)
@@ -159,7 +159,7 @@
                 # Header's got lots of smarts, so use it.
                 print >> self._fp, Header(
                     v, maxlinelen=self._maxheaderlen,
-                    header_name=h, continuation_ws='\t').encode()
+                    header_name=h).encode()
         # A blank line always separates headers from body
         print >> self._fp