Run payload procs in spec context

This allows you to reference spec variables in your comparison:

```ruby
let(:bar) { 'hello!' }

assert_payload(:foo, record, json_item) do
  key(:something) { bar } # bar is now available
end
```

Author: Lee Richmond

lib/jsonapi_spec_helpers.rb

@@ -34,7 +34,8 @@ def assert_payload(name, record, json, &blk)
         prc = options[:proc]
         if (expect(json).to have_payload_key(attribute, options[:allow_nil])) == true
           unless options[:allow_nil]
-            expect(json[attribute.to_s]).to match_payload(attribute, prc.call(record))
+            output = instance_exec(record, &prc)
+            expect(json[attribute.to_s]).to match_payload(attribute, output)
 
             if options[:type]
               expect(json[attribute.to_s]).to match_type(attribute, options[:type])

spec/assert_payload_spec.rb

@@ -37,17 +37,30 @@
 
     context 'when payload is valid' do
       let(:post_record) do
-        double \
+        attrs = {
           id: 1,
           title: 'post title',
           description: 'post description',
           views: 100
+        }
+        double(attrs).as_null_object
       end
 
+      let(:some_let_var) { 'foo' }
+
       it 'passes assertion' do
         assert_payload(:post, post_record, json_item)
       end
 
+      it 'has access to spec context' do
+        post_record.title = some_let_var
+        item = json_item
+        item['title'] = some_let_var
+        assert_payload(:post, post_record, item) do
+          key(:title, String) { some_let_var }
+        end
+      end
+
       context 'when json value matches payload value, but wrong type' do
         before do
           json['data']['attributes']['views'] = '100'