Fixed parsing for zero content length response

[infohash]

The content-type in the header is optional when the payload has zero content length. See, Should Content-Type header be present when the message body is empty?.

This happens when the request to IdP is made for revoking tokens. In that case, IdP returns a 200 OK status code with no body and no Content-Type in the response header.

See, About the revoke request.

Revoking a token that is invalid, expired, or already revoked returns a 200 OK status code to prevent any information leaks.

To parse responses with zero content length, just check if the value of content-length is 0 and return the status code if it does.

[codecov-commenter]

Codecov Report

Merging #812 (9bfbd75) into master (62c7318) will increase coverage by 0.12%.
The diff coverage is 60.00%.

❗ Current head 9bfbd75 differs from pull request most recent head a90a2c0. Consider uploading reports for the commit a90a2c0 to get more accurate results

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
+ Coverage   63.72%   63.84%   +0.12%     
==========================================
  Files          64       64              
  Lines       11856    11860       +4     
  Branches     2411     2413       +2     
==========================================
+ Hits         7555     7572      +17     
+ Misses       3703     3686      -17     
- Partials      598      602       +4     

Impacted Files	Coverage Δ
src/oic/oauth2/util.py	71.66% <50.00%> (+0.48%)	⬆️
src/oic/oauth2/__init__.py	69.69% <66.66%> (+0.40%)	⬆️
src/oic/extension/client.py	46.57% <0.00%> (+7.53%)	⬆️

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[tpazderka]

The new code should be covered by tests.

[tpazderka]

The tests should be unittests, so you should write a test for parse_request_response and verify_header functions that cover the changes introduced here.

[infohash]

@tpazderka Awaiting review.

[infohash]

When will be the new version released? zamzterz/Flask-pyoidc/#801 depends on it.