Make sure the PoP handler is capable of validating the 'b' claim

[mafurman]

Please provide us with the following information:

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Minimal steps to reproduce

The signed http request sent in the PoP sample does not have a 'b' (body) claim, so this claim isn't checked during the validation process.

Expected/desired behavior

We need to make sure that the body extracted by the handler can be properly validated.

Mention any other details that might be useful

We also want to make sure we are using an appropriately sized buffer when reading in the body of the request inside the handler.
NOTE: Validation of the 'b' claim is optional and should not be added to the sample, but we should confirm that it works.