lib-sieve: made sure that modified messages are not stored in the source...

lib-sieve: made sure that modified messages are not stored in the source mailbox when it was opened read-only.
This is currently only relevant for the sieve-filter tool to avoid duplicating messages in the
source folder. This can happen when the messages are modified (e.g. by the editheader extension) and the
source folder is opened read-only, thus preventing deletion of the original message.