Better error message for missing let

I've run into this problem a number of times. The original error message
is actually a `NoMethodError` and it's not always immediately clear what
the solution is, particuarly if you're new to rspec or rswag

Wanted to see if this kind of behavior is something that the rswag team
would be interested in adopting. I have a few other of these in mind
(will do as small PRs, with tests)

rswag-specs/lib/rswag/specs/request_factory.rb

@@ -194,12 +194,36 @@ module Rswag
 
       def build_json_payload(parameters, example)
         body_param = parameters.select { |p| p[:in] == :body }.first
-        body_param ? example.send(body_param[:name]).to_json : nil
+
+        return nil unless body_param
+
+        raise(MissingParameterError, body_param[:name]) unless example.respond_to?(body_param[:name])
+
+        example.send(body_param[:name]).to_json
       end
 
       def doc_version(doc)
         doc[:openapi] || doc[:swagger] || '3'
       end
     end
+
+    class MissingParameterError < StandardError
+      attr_reader :body_param
+
+      def initialize(body_param)
+        @body_param = body_param
+      end
+
+      def message
+        <<~MSG
+          Missing parameter #{body_param}
+
+          Please check your spec. It looks like you defined a body parameter,
+          but did not declare usage via let. Try adding:
+
+              let(:#{body_param}) {}
+        MSG
+      end
+    end
   end
 end

rswag-specs/spec/rswag/specs/request_factory_spec.rb

@@ -160,6 +160,21 @@ module Rswag
             end
           end
 
+          context 'missing body parameter' do
+            before do
+              metadata[:operation][:parameters] = [{ name: 'comment', in: :body, schema: { type: 'object' } }]
+              allow(example).to receive(:comment).and_raise(NoMethodError, "undefined method 'comment'")
+              allow(example).to receive(:respond_to?).with(:'Content-Type')
+              allow(example).to receive(:respond_to?).with('comment').and_return(false)
+            end
+
+            it 'uses the referenced metadata to build the request' do
+              expect do
+                request[:payload]
+              end.to raise_error(Rswag::Specs::MissingParameterError, /Missing parameter comment/)
+            end
+          end
+
           context 'form payload' do
             before do
               metadata[:operation][:consumes] = ['multipart/form-data']