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.

Ok bad example.  What about unexpected user agents like "lynx" or "ruby"?
 On Nov 24, 2014 7:08 PM, "Alberto Bellotti" <notifications@github.com>
wrote:

> In vmdb/spec/requests/auth_spec.rb:
>
> > @@ -39,13 +39,13 @@
> >    context 'w/ a valid session' do
> >      it "allows access w/ a valid referer" do
> >        post '/dashboard/authenticate', :user_name => 'admin', :user_password => 'smartvm'
> > -      get '/ems_cloud/show_list', nil, 'HTTP_REFERER' => "http://www.example.com/"
> > +      get '/ems_cloud/show_list', nil, 'HTTP_USER_AGENT' => "chrome", 'HTTP_REFERER' => "http://www.example.com/"
>
> REST API authentication is via /api, not the above. For the above, without
> HTTP_USER_AGENT, I was getting an undefined method downcase for nil for
> ie8_referer_exception?
> allowed_access->referer_valid->ie8_referer_exception?
>
> —
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> <https://github.com/ManageIQ/manageiq/pull/1116/files#r20833565>.
>