webhook pipeline headers

I created a webhook pipeline endpoint (for use with paypal payflow link silent post to get me the results of a successful transaction).   I am trying to capture the headers of the incoming request but putting "{{a.headers}}" is not working, presumably because it's an actual array.  I want to loop through the headers, concat them into one string, and paste that into a text field when I call create record.    How do I do this?