fix(example): set content-type in the userinfo response

[muhlemmer]

This change sets the content-type header to application/json for the response sent to the browser in the app example. This enables pretty-printing of the userinfo json document in at least Chromium.

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[github-actions[bot]]

:tada: This PR is included in version 3.25.1 :tada:

The release is available on GitHub release

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