Merge branch 'test-fix' into 'master'

Fix bug with non-HTTP Shib sessions being valid for HTTP sessions

Previously, the `PreauthUserDetailsProvider` was only checking that a
valid Shib session existed by looking for the regular or HTTP Shib
session header. This check is now strengthened by validating the
correct header exists for the correct instance.

Also renamed and changed some things in the `Preauth` test--it's now called `HTTPPreauthUserDetailsProviderTest` to reflect the fact that it's only testing the HTTP version of `Preauth`. Along those same lines, the test user now uses the HTTP headers and has been renamed to `testuser_http.json`.

Please review: @ahoffmann 

See merge request !7