Report on the content type of the remote API when event verify-subscription-ing

[BarryCarlyon]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

Add a third test to the event sub verifiy function, to report what the content type is of the returned response

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)

[lleadbet]

ezpz, lgtm.