Add support for checking response type before adding headers

[juunas11]

Quoting dasmulli on Slack:

could also look into not adding the headers before the next steps of the pipeline, but rather registering a delegate somewhere in the response. this would enable for response type checking.. other than checking the request for known api paths etc.

[agilenut]

FYI - I think the change proposed here allows you to check the content-type of the response in the OnSendingHeader callback.

https://github.com/juunas11/aspnetcore-security-headers/issues/39