The signature for send_message included mutable defaults for two sets of
options, mail_options and rcpt_options. Additionally, the default for
rcpt_options was a dictionary, not a list. These have now been changed
to have a default of None, which is set to a dictionary during function
execution.

documentation changed to reflect the new defaults for `mail_options` and
`rcpt_options` in `SMTP.send_message`.