Set Content-Type for backchannel logout POSTs

[rohe]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.
• New code is annotated.
• Changes are covered by tests.

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According to spec the POST body uses the application/x-www-form-urlencoded encoding.

[tpazderka]

You need to run black on the code in order for quality to pass.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f67df35). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #712   +/-   ##
=========================================
  Coverage          ?   63.83%           
=========================================
  Files             ?       63           
  Lines             ?    11684           
  Branches          ?     2053           
=========================================
  Hits              ?     7459           
  Misses            ?     3656           
  Partials          ?      569

Impacted Files	Coverage Δ
src/oic/oic/provider.py	74.78% <100%> (ø)

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[tpazderka]

Failing CI is because of #714, will be dealt with there...