Use method_name instead of method when generating required warnings

Unlike the previous commit, this isn't strictly required as we're not calling
the un-delegated `method`, but instead the warning will contain a standard
`to_s` of the `@controller_method` that isn't very easy to read, like this:

    WARNING (105): [#<Apipie::MethodDescription:0x0000000126316f60>] -- The
    parameter :param is optional but default value is not specified (use
    :default_value => ...)

By using `method_name` instead we get symmetry with the hash warning from the
previous commit.

lib/apipie/generator/swagger/param_description/builder.rb

@@ -98,8 +98,8 @@ def required_from_path?
   def warn_optional_without_default_value(definition)
     if !required? && !definition.key?(:default)
       method_id =
-        if @param_description.is_a?(Apipie::ResponseDescriptionAdapter::PropDesc)
-          @controller_method
+        if @controller_method.present? && @param_description.is_a?(Apipie::ResponseDescriptionAdapter::PropDesc)
+          @controller_method.method_name
         else
           Apipie::Generator::Swagger::MethodDescription::Decorator.new(@controller_method).ruby_name
         end

spec/lib/apipie/generator/swagger/param_description/builder_spec.rb

@@ -145,6 +145,24 @@
             /is optional but default value is not specified/
           ).to_stderr
         end
+
+        context 'and param is a prop desc with a delegated controller method' do
+          let(:param_description) do
+            Apipie.prop(:param, 'object', param_description_options, [])
+          end
+
+          let(:method_desc) do
+            Apipie::Generator::Swagger::MethodDescription::Decorator.new(
+              Apipie::MethodDescription.new(:show, resource_desc, dsl_data)
+            )
+          end
+
+          it 'warns' do
+            expect { subject }.to output(
+              /is optional but default value is not specified/
+            ).to_stderr
+          end
+        end
       end
     end
   end