gh-XXXXX: Fix email.utils.collapse_rfc2231_value crash on non-3-tuples

Passing a tuple of length != 3 fell through to unquote(value) which
calls .startswith() on the tuple, raising AttributeError. Handle
non-3-tuples explicitly before calling unquote.

Author: stefanzetzsche

Lib/email/utils.py

@@ -449,6 +449,8 @@ def decode_params(params):
 def collapse_rfc2231_value(value, errors='replace',
                            fallback_charset='us-ascii'):
     if not isinstance(value, tuple) or len(value) != 3:
+        if isinstance(value, tuple):
+            return str(value)
         return unquote(value)
     # While value comes to us as a unicode string, we need it to be a bytes
     # object.  We do not want bytes() normal utf-8 decoder, we want a straight