fix crash caused by incorrect query in scope

setting all categories to be secured led to a blank screen on all pages

use stabby lambda for consistency in class

make the test a little more concise

- move the local assignments into let blocks for
reusability

- remove calls to `to_a`, which aren't needed

- use 'be_empty' instead of '[]' to be consistent
with the other matchers in the test

add a test for the `secured` scope with multiple
secured categories
Merge pull request #849 from leemachin/fix-secure-categories-query

Fix error in secured categories scope