Clear up Person vs. EagerPerson when testing vault_persist_before_save!

We use `Person` in the "when not used" context because that model class
has not had `vault_persist_before_save!` called on it.  `EagerPerson` has
and that's why we use it in the "when used" context.  It'd be clearer if
we could explicitly call it in the spec, but there's no clean up option to
unset it so we need separate models.

spec/unit/encrypted_model_spec.rb

@@ -91,8 +91,11 @@
 
   describe '#vault_persist_before_save!' do
     context "when not used" do
+      # Person hasn't had `vault_persist_before_save!` called on it
+      let(:model_class) { Person }
+
       it "the model has an after_save callback" do
-        save_callbacks = Person._save_callbacks.select do |cb|
+        save_callbacks = model_class._save_callbacks.select do |cb|
           cb.filter == :__vault_persist_attributes!
         end
 
@@ -104,34 +107,37 @@
         expect(persist_callback.kind).to eq :after
       end
 
-      it 'calls the correnct callback' do
-        person = Person.new(ssn: '123-45-6789')
-        expect(person).to receive(:__vault_persist_attributes!)
+      it 'calls the correct callback' do
+        record = model_class.new(ssn: '123-45-6789')
+        expect(record).to receive(:__vault_persist_attributes!)
 
-        person.save
+        record.save
       end
 
       it 'encrypts the attribute if it has been saved' do
-        person = Person.new(ssn: '123-45-6789')
+        record = model_class.new(ssn: '123-45-6789')
         expect(Vault::Rails).to receive(:encrypt).with('transit', 'dummy_people_ssn', anything, anything, anything).and_call_original
 
-        person.save
+        record.save
 
-        expect(person.ssn_encrypted).not_to be_nil
+        expect(record.ssn_encrypted).not_to be_nil
       end
     end
 
     context "when used" do
+      # EagerPerson has had `vault_persist_before_save!` called on it
+      let(:model_class) { EagerPerson }
+
       it "the model does not have an after_save callback" do
-        save_callbacks = EagerPerson._save_callbacks.select do |cb|
+        save_callbacks = model_class._save_callbacks.select do |cb|
           cb.filter == :__vault_persist_attributes!
         end
 
         expect(save_callbacks.length).to eq 0
       end
 
       it "the model has a before_save callback" do
-        save_callbacks = EagerPerson._save_callbacks.select do |cb|
+        save_callbacks = model_class._save_callbacks.select do |cb|
           cb.filter == :__vault_encrypt_attributes!
         end
 
@@ -144,20 +150,20 @@
       end
 
       it 'calls the correct callback' do
-        eager_person = EagerPerson.new(ssn: '123-45-6789')
-        expect(eager_person).not_to receive(:__vault_persist_attributes!)
-        expect(eager_person).to receive(:__vault_encrypt_attributes!)
+        record = model_class.new(ssn: '123-45-6789')
+        expect(record).not_to receive(:__vault_persist_attributes!)
+        expect(record).to receive(:__vault_encrypt_attributes!)
 
-        eager_person.save
+        record.save
       end
 
       it 'encrypts the attribute if it has been saved' do
-        eager_person = EagerPerson.new(ssn: '123-45-6789')
+        record = model_class.new(ssn: '123-45-6789')
         expect(Vault::Rails).to receive(:encrypt).with('transit', 'dummy_people_ssn',anything,anything,anything).and_call_original
 
-        eager_person.save
+        record.save
 
-        expect(eager_person.ssn_encrypted).not_to be_nil
+        expect(record.ssn_encrypted).not_to be_nil
       end
     end
   end