Default fields resource option

Add an option to the resource class to specify the default fields. Would be used by the request parser if the fields are not specified. Would allow a pared down fields set as the default, while still allowing the full field set if requested.

For example:

class CommentResource < JSONAPI::Resource
  attributes :body
  has_one :post
  has_one :author, class_name: 'Person'
  has_many :tags

  filters :body

  default_fields [:body, :post, :author]
end

Would skip the tags relationship, unless specified in the request.

Comments?

首先定位资源类选项处理以及 issue 中提到的请求解析器。跟踪请求的字段和关系是如何被选择的，然后确定默认列表应如何仅在字段缺失时应用。完成的标准是资源可以声明默认字段，被省略的关系默认排除，并且显式请求的字段仍然有效。