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

> 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.