parser: Handle multiple reference headers

parser: Handle multiple reference headers

It's possible to duplicate message headers multiple times. One common
case is the 'Received' header, but it appears that multiple
'In-Reply-To' and 'References' headers are also a thing.

Handle these cases through the use of the 'Message.get_all' function,
which returns all matching headers, instead of the 'Message.get'
function previously used.

Signed-off-by: Stephen Finucane <stephen@that.guru>