Fixed spec/requests/ authentication specs

- Update session_store to use :memory instead of default cookie_store for tests, eliminating the 4K limit that was giving us the 500's.
- Needed to specify HTTP_USER_AGENT for a couple of tests in auth_spec.rb

This fix is needed before enabling spec:requests as part of test:vmdb for the REST API.

@martinpovolny @Fryguy please review.

Why the need for the web server worker?  Can we just say:

```ruby
if Rails.env.test?
  Vmdb::Application.config.session_store :memory_store
else
```

Also, why is this needed?  Is there a limit on memcache?