Fixing an instance where http handlers could change the header state of a stored request.

Prior to this fix, a request could be stored, and if it were replayed in the same tape session, it would add an X-Betamax header to the stored request. In this case, the tape would be written with these X-Betamax headers in them, and in my particular use case, would result in the occasional parse error.

The YAML should never contain any of the X-Betamax headers when persisted.