The OIDC code was written as a GSOC by Enrico Carbognani while he was an undergraduate
at the University of Luxembourg who goes by naccht. A blog entry by the author:

 https://blogs.python-gsoc.org/en/nacchts-blog/weekly-check-in-week-9-22-07-28-07/

says:

  I had issues getting the headers in the GET request in Roundup Actions. As the
  documentation doesn't have much on this regard and as I couldn't find similar
  pieces of code I had to study Roundup's source code to understand how to achieve it.


I am not sure if he was referring to accessing the response headers from a get or adding 
additional header to a get. I didn't find any examples of the former, but he has
a few references to:

  self.client.additional_headers['Content-Type'] = ...

in openid_login so that might be the reference. There are no hit on additional_headers
in the doc directory.