Belt-and-suspenders specs of Post attributes.

Also grouping and DRYing up Post specs.

Now caught up with http://objectsonrails.com/#ID-602d83bb-6946-4afd-9dfe-ce24faabe5cd.

RSpec: 44 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 19 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4