stub! doesn't work with any_instance.

See https://github.com/rspec/rspec-mocks/issues/57 . Between RSpec
2.12 and 2.6 using stub! with any_instance started throwing
exceptions. However, according to that ticket it never actually
worked. So this stub line was unnecessary. (It if were necessary, the
test would fail without it.)
Rephrase specs to avoid using `empty?`.

These specs started failing when RSpec was updated to 2.12.0 from
2.6.1. FWIW, It would pass up to 2.10.1 and fail on 2.11.0.

Closer inspection, however, shows that based on AR's behavior they
should have always failed. Calling `empty?` on a has_many :through
assocation that has not been loaded yet always looks at the database.
This is true even if `.build` has been called on the association to
add something to the collection without saving it.

Demo gist: https://gist.github.com/rsutphin/4aa38ef2f44236a4b6a2

So the real question is, why did this spec ever pass? There must be
some behavior difference between RSpec 2.10 and 2.11 that resulted in
the association being loaded, making `empty?` work correctly. Much
trawling through diffs for rspec-rails, rspec-expectations, and rspec-
core did not yield any useful hypotheses about what this change might
be. Since it's not an RSpec bug, though, I'm satisfied to just
rephrase the specs in an equivalent way that does result in the
association being loaded.