Bug 1070429 - [email/IMAP] premature saving leads to missing bodies

`Object.create(null)` satisfies the concern for me. My concern was mostly in the future if Object.prototype ever gets a property that is an array or something, like a string, where args[0] gives a truthy value, this code will break. So more of a concern about future language changes vs. us using something that adds chaos monkeys to built in prototypes.