api: Show all headers with the same key

api: Show all headers with the same key

While the code on our side returns all (key, value) pairs for email
headers, Django's REST framework probably uses dictionaries behind the
scenes. This means that having multiple headers with same key (eg
'Received', which is totally valid and common situation), only one of
these headers is visible in the REST API.

Let's hack around this by returning a list of values in case the key is
present multiple times.

Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>