authz: Add Content-Type header in the call to Keto

[Sbou]

Related issue

#289 @aeneasr

Proposed changes

This change add a Content-Type header when Oathkeeper calls Keto.
As Keto expects a Json body, I added this header:
Content-Type: application/json

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the
  developer guide (if appropriate)

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Awesome, thanks!