Issue #1670765: Prevent email.generator.Generator from re-wrapping

headers in multipart/signed MIME parts, which fixes one of the sources of
invalid modifications to such parts by Generator.  Patch and tests by
Martin von Gagern.

Lib/email/generator.py

@@ -237,6 +237,17 @@ class Generator:
             print >> self._fp
             self._fp.write(msg.epilogue)
 
+    def _handle_multipart_signed(self, msg):
+        # The contents of signed parts has to stay unmodified in order to keep
+        # the signature intact per RFC1847 2.1, so we disable header wrapping.
+        # RDM: This isn't enough to completely preserve the part, but it helps.
+        old_maxheaderlen = self._maxheaderlen
+        try:
+            self._maxheaderlen = 0
+            self._handle_multipart(msg)
+        finally:
+            self._maxheaderlen = old_maxheaderlen
+
     def _handle_message_delivery_status(self, msg):
         # We can't just write the headers directly to self's file object
         # because this will leave an extra newline between the last header