Fix spec

While working on this branch, Kito found that this test fails due to:

https://github.com/rails/rails/commit/6ec8ba16d85d5feaccb993c9756c1edcbbf0ba13#diff-fdcf8b65b5fb954372c6fe1ddf284c78R76

We are not yet sure if it's a bug in paperclip or in Rails itself. With
current `ActiveModel::Errors` implementation the following happens:

```
record.errors # => @messages = {}
record.errors.include?(:foo) # => false
record.errors # => @messages = { :foo => [] }
```

Which bit us in:
https://github.com/thoughtbot/paperclip/blob/69f18375333234b6f395300266e2612936bd242e/lib/paperclip/validators/attachment_file_name_validator.rb#L23

Another related Rails commit:
https://github.com/rails/rails/commit/b97035df64f5b2f912425c4a7fcb6e6bb3ddab8d

I worked around the issue by changing what we assert in this spec. I am
still not sure that this is a bug in current Rails master.

cc @kitop for review

spec/paperclip/validators_spec.rb

@@ -64,12 +64,11 @@ describe Paperclip::Validators do
       assert_raises(RuntimeError){ dummy.valid? }
     end
 
-    it 'allows you to attach a file that does not violates these validations' do
+    it 'allows you to attach a file that does not violate these validations' do
       dummy = Dummy.new(avatar: File.new(fixture_file('rotated.jpg')))
-      expect(dummy.errors.keys).to match_array []
+      expect(dummy.errors.full_messages).to be_empty
       assert dummy.valid?
     end
-
   end
 
   context "using the helper with a conditional" do