Don't raise KeyError when trying to navigate to nil next_page

[KevinBongart]

I'm having trouble looping through paginated resources because the last page doesn't have a next link, and instead of resource.pages.next returning nil, it raises a KeyError.

resources = Api::Resource.paginated(per_page: 10)

while resources.present?
  resources.map do |resource|
    puts resouce.id
  end

  resources = begin
    resources.pages.next
  rescue KeyError
    nil
  end
end

This PR attempts to fix it, but there are many questions raised from investigating:

1. Is it the responsibility of the gem or the user to handle the last page? Users currently have to rescue a KeyError exception, which isn't explicitly related to a missing resource.
2. If the gem can make it easier for the user, should it return nil? []? an empty Resource object?
3. What about the previous page on the very first page of a resource?

Or… am I iterating through pages the wrong way? Happy to complete this PR or add more documentation either way!

/cc @chingor13 @bvandenbos @sharshenov @reidab @dplummer

[KevinBongart]

I ended up implementing a enumerator on top of the pagination system, based on @rossta's work (https://rossta.net/blog/paginated-resources-in-ruby.html)

class JsonApiClientEnumerator
  include Enumerable

  def initialize(resource, per_page: nil)
    @resource = resource.paginate(per_page: per_page)
    @paginator = nil
    @collection = []
  end

  def each(start = 0)
    return to_enum(:each) unless block_given?

    @collection[start..-1].each do |element|
      yield(element)
    end

    unless @last_response_empty
      start = [@collection.size, start].max
      fetch_next_page
      each(start, &Proc.new)
    end
  end

  private

  def fetch_next_page
    result_set = next_page_result_set

    if result_set.present?
      @collection += result_set
    else
      @last_response_empty = true
    end
  end

  def next_page_result_set
    @paginator = if @paginator.blank?
                   # first iteration: get the pages from the resource
                   @resource.pages
                 else
                   # next iterations: get the pages from the paginator object
                   @paginator.next.pages
                 end

    @paginator.result_set
  rescue KeyError
    # https://github.com/chingor13/json_api_client/pull/203
    []
  end
end

It can be used to lazily iterate through a paginated resource:

JsonApiClientEnumerator.new(CoolDog).each do |cool_dog|
  puts cool_dog.name
end

And customize the pagination limit:

JsonApiClientEnumerator.new(CoolDog, per_page: 5)

[KevinBongart]

@chingor13 Would it be useful if I added this enumerator class to json_api_client?

[chingor13]

I feel like fixing this bug when looping is good enough for now. I think returning nil is fine for now. If you return an empty result set, you may still have to add a check for an empty result set or loop indefinitely.