bpo-40837: Fix email.utils.encode_rfc2231(string, None, None)

encode_rfc2231() must not change the returned value if no transformation of the input was done.
This is also mentioned in the docstring of that function.

Actual behavior:
encode_rfc2231('foo bar', None, None)
'foo%20bar'

Expected behavior:
encode_rfc2231('foo bar', None, None)
'foo bar'

Author: spaceone

Lib/email/utils.py

@@ -243,9 +243,9 @@ def encode_rfc2231(s, charset=None, language=None):
     charset is given but not language, the string is encoded using the empty
     string for language.
     """
-    s = urllib.parse.quote(s, safe='', encoding=charset or 'ascii')
     if charset is None and language is None:
         return s
+    s = urllib.parse.quote(s, safe='', encoding=charset or 'ascii')
     if language is None:
         language = ''
     return "%s'%s'%s" % (charset, language, s)