In a job registration request, ensure that the request namespace "header" and job
namespace field match.  This should be the case already in prod, as http
handlers ensures that the values match [1].

This mitigates bugs that exploit bugs where we may check a value but act
on another, resulting into bypassing ACL system.

[1] https://github.com/hashicorp/nomad/blob/v0.9.5/command/agent/job_endpoint.go#L415-L418