Add pending code to flip-flop results of specs.

This isn't the cleanest approach, but should be good for now.

Obviously, there are two contexts for these specs: one is from the
maintainer's standpoint, the other is from the trainee who is using
RailsGoat for training.

The maintainer wants all of these specs to pass, to ensure the
vulnerabilities are still functional as vulnerabilities.

The trainee could potentially use these specs (though reading the specs
contains spoilers) to track and verify their fixes.

I've wired in a pending block around each assertion that checks a method
to see what the result of the pending call would be. You can see
examples of how this works with conditions here:
https://www.relishapp.com/rspec/rspec-core/v/2-14/docs/pending/pending-examples

This means these specs will all fail now by default (the trainee
context), but will pass, when vulnerable, if the RAILSGOAT_MAINTAINER
env var is set.

The only flaw at the moment is that in the trainee context, fixing the
vulnerabilities will result in the specs going from failing to
_pending_, not passing (which makes sense, given how we're using RSpec's
pending functionality).

Maybe it'd be simpler/better to have a boolean toggle of our own somehow
wrap the assertions in blocks to do explicitly what we want (flip-flop
the result based on the context).