RestRequestException when using custom headers

[methsib]

Our tests started failing today with following exception and further investigations found that latest version of requests is causing the issue.

requests.exceptions.InvalidHeader: Header part ('application/x-www-form-urlencoded') from {'content-type': 'application/x-www-form-urlencoded'} must be of type str or bytes, not <class 'tavern.util.formatted_str.FormattedString'>

Steps to reproduce

---
test_name: Test
stages:
  - name: Test requests issue
    request:
      url: "http://www.example.com/get_info"
      method: GET
      headers:
        content-type: application/x-www-form-urlencoded 

Error message

Errors:
E   tavern.util.exceptions.RestRequestException
---------------------------------------------------------------------------------------------------------------------------------------- Captured log call ----------------------------------------------------------------------------------------------------------------------------------------- 
ERROR    tavern._plugins.rest.request:request.py:552 Error running prepared request
Traceback (most recent call last):
  File "C:\myenv\lib\site-packages\tavern\_plugins\rest\request.py", line 550, in run
    return self._prepared()
  File "C:\myenv\lib\site-packages\tavern\_plugins\rest\request.py", line 530, in prepared_request
    return session.request(**self._request_args)
  File "C:\myenv\lib\site-packages\requests\sessions.py", line 573, in request
    prep = self.prepare_request(req)
  File "C:\myenv\lib\site-packages\requests\sessions.py", line 484, in prepare_request
    p.prepare(
  File "C:\myenv\lib\site-packages\requests\models.py", line 369, in prepare
    self.prepare_headers(headers)
  File "C:\myenv\lib\site-packages\requests\models.py", line 491, in prepare_headers
    check_header_validity(header)
  File "C:\myenv\lib\site-packages\requests\utils.py", line 1037, in check_header_validity
    raise InvalidHeader(
requests.exceptions.InvalidHeader: Header part ('application/x-www-form-urlencoded') from {'content-type': 'application/x-www-form-urlencoded'} must be of type str or bytes, not <class 'tavern.util.formatted_str.FormattedString'>

Notes

It seems the headers are being turned into FormattedString objects and called in the rest module here.
And the headers specifically are added to the request args here.

[raidgar98]

quick fix:

during installation force dependency

pip install requests==2.27.1 tavern

[Alonreznik]

Also here, and worked.

[michaelboulton]

Should be fixed in 1.23.2.