Fix test to avoid memoization bug

I don't believe primary_key is ever changed within a request

spec/bullet/ext/object_spec.rb

@@ -26,21 +26,21 @@
     end
 
     it 'should return primary key value' do
-      post = Post.first
       Post.primary_key = 'name'
+      post = Post.first
       expect(post.bullet_primary_key_value).to eq(post.name)
       Post.primary_key = 'id'
     end
 
     it 'should return value for multiple primary keys from the composite_primary_key gem' do
-      post = Post.first
       allow(Post).to receive(:primary_keys).and_return(%i[category_id writer_id])
+      post = Post.first
       expect(post.bullet_primary_key_value).to eq("#{post.category_id},#{post.writer_id}")
     end
 
     it 'should return value for multiple primary keys from ActiveRecord 7.1' do
-      post = Post.first
       allow(Post).to receive(:primary_key).and_return(%i[category_id writer_id])
+      post = Post.first
       expect(post.bullet_primary_key_value).to eq("#{post.category_id},#{post.writer_id}")
     end