Add coverage for Params

The sql_value extension point was not used, so coverage
dropped. Params is context-independent, so it's likely that it will be
pulled out of Permits soon; we might as well test it now.

spec/checkpoint/permits_spec.rb

@@ -67,3 +67,42 @@ def resource(type: 'resource', id: 1)
     Checkpoint::Resource.new(entity)
   end
 end
+
+RSpec.describe Checkpoint::Permits::Params do
+  context 'with an array of items' do
+    let(:item1)  { double('item', to_s: 'item1') }
+    let(:item2)  { double('item', to_s: 'item2') }
+    let(:items)  { [item1, item2] }
+    let(:params) { described_class.new(items, 'prefix') }
+
+    it 'gives an array with both placeholders, using the prefix' do
+      expect(params.placeholders).to contain_exactly(:$prefix_0, :$prefix_1)
+    end
+
+    it 'gives a hash-convertible array with both values' do
+      expect(params.values).to contain_exactly([:prefix_0, 'item1'], [:prefix_1, 'item2'])
+    end
+  end
+
+  context 'with a single item' do
+    let(:item)   { double('item', to_s: 'item') }
+    let(:params) { described_class.new(item, 'prefix') }
+
+    it 'gives an array of one placeholder, using the prefix' do
+      expect(params.placeholders).to eq [:$prefix_0]
+    end
+
+    it 'gives a hash-convertible array of one value' do
+      expect(params.values).to eq [[:prefix_0, 'item']]
+    end
+  end
+
+  context 'with an item implementing sql_value' do
+    let(:item)   { double('item', sql_value: 'sql_value') }
+    let(:params) { described_class.new(item, 'prefix') }
+
+    it 'uses the #sql_value and not #to_s' do
+      expect(params.values).to eq [[:prefix_0, 'sql_value']]
+    end
+  end
+end