email: do not fold WhiteSpaceTerminal not starting with a WSP

e.g. with a 40 characters length policy, the below
`((loremipsum)(loremipsum)(loremipsum)(loremipsum))<spy@example.org>`

should not be folded to
`((loremipsum)(loremipsum)(loremipsum)\n(loremipsum))<spy@example.org>`

but should rather stay
`((loremipsum)(loremipsum)(loremipsum)(loremipsum))<spy@example.org>`

to prevent to add a new line not starting with a space / with no
indentation

Author: beledouxdenis

Lib/email/_header_value_parser.py

@@ -949,7 +949,7 @@ def value(self):
         return ' '
 
     def startswith_fws(self):
-        return True
+        return self and self[0] in WSP
 
 
 class ValueTerminal(Terminal):