Add header at top with email.message
the EmailMessage class of email.message provides the methods
add_header() and __setitem__() to add a header to a message.
add_header() effectively calls __setitem__(), which does
`self._headers.append(self.policy.header_store_parse(name, val))`. This
inserts the header at the bottom.
It is, however, sometimes desired to insert a new header at the top of
an (existing) message. This API doesn?t directly allow this. In my
opinion, add_header() should have a flag at_top=False or similar, so
that one can get this behaviour (it?ll be a bit difficult with
__setitem__). What do you think about this? Is there a feasible way to
do this and change the library? Should I post it somewhere where the
devs can hear it and suggest that?