Fix undefined methods

[mcauser]

1. In Operation def definitions was building a definitions hash by id, but not actually returning the hash for chaining. Resulting in error:

undefined method `[]=' for #<Swaggard::Swagger::Definition:0x00007feae308c048 @id="Api::V1::CustomersController.create_response", @title="", @properties=[#<Swaggard::Swagger::Response::ResponseModel:0x00007feae308c318 @is_array_response=nil, @response_class="CustomerSerializer", @id="customer">], @description="", @ancestors=[], @ignore_inherited=false>

2. In Definition def to_doc was selecting required properties, but all_properties contained a ResponseModel which didn't have a required? method.

undefined method `required?' for #<Swaggard::Swagger::Response::ResponseModel:0x00007fc65125fbb0 @is_array_response=nil, @response_class="CustomerSerializer", @id="customer">

Adding the missing required? method seemed cleaner than checking each for the presence of the method before calling it.

# existing
required_properties = all_properties.select(&:required?).map(&:id)
# meh
required_properties = all_properties.select { |p| p.respond_to?(:required?) && p.required? }.map(&:id)

On Ruby 3.0.6 / Rails 6.1.7.4