Addition of ARC-Seal header

[Andrew Beverley <andy@xxxxxxxxxxx>]

Hi,

I am trying to fix what appears to be a bug, whereby previous ARC 
signatures are not included in the ARC-Seal that OpenARC produces.

I am struggling to find exactly where in the source code the headers are 
retrieved for the ARC-Seal signature. Can anyone point me in the right 
direction please?

I have found what appears to be the use of arc_b64sig to add the 
signature, but I cannot work out where the data for this is set.

Grateful for any help!

Andy