Make sure PoP handler is capable of handling signed http requests with a varying number of spaces

> 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
> PoP handler currently only works when there's only one space present between "PoP" and the signed http request.

### Any log messages given by the failure
> The handler will be unable to properly extract the signed http request from the authorization header.

### Expected/desired behavior
> The handler should work regardless of the number of spaces.