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.

we were getting 500's with the default 4k limit with cookie_store, ye-old CookieOverflow errors.