Make paginator page/per_page param keys configurable (#239)

Author: AMHOL

README.md

@@ -447,6 +447,17 @@ end
 
 You can customize how your resources find pagination information from the response.
 
+If the [existing paginator](https://github.com/chingor13/json_api_client/blob/master/lib/json_api_client/paginating/paginator.rb) fits your requirements but you don't use the default `page` and `per_page` params for pagination, you can customise the param keys as follows:
+
+```ruby
+JsonApiClient::Paginating::Paginator.page_param = "page[number]"
+JsonApiClient::Paginating::Paginator.per_page_param = "page[size]"
+```
+
+Please note that this is a global configuration, so library authors should create a custom paginator that inherits `JsonApiClient::Paginating::Paginator` and configure the custom paginator to avoid modifying global config.
+
+If the [existing paginator](https://github.com/chingor13/json_api_client/blob/master/lib/json_api_client/paginating/paginator.rb) does not fit your needs, you can create a custom paginator:
+
 ```ruby
 class MyPaginator
   def initialize(result_set, data); end

lib/json_api_client/paginating/paginator.rb

@@ -1,11 +1,18 @@
 module JsonApiClient
   module Paginating
     class Paginator
+      class_attribute :page_param,
+                      :per_page_param
+
+      self.page_param = "page"
+      self.per_page_param = "per_page"
+
       attr_reader :params, :result_set, :links
+
       def initialize(result_set, data)
         @params = params_for_uri(result_set.uri)
         @result_set = result_set
-        @links = data['links']
+        @links = data["links"]
       end
 
       def next
@@ -28,7 +35,7 @@ def total_pages
         if links["last"]
           uri = result_set.links.link_url_for("last")
           last_params = params_for_uri(uri)
-          last_params.fetch("page") do
+          last_params.fetch(page_param) do
             current_page
           end.to_i
         else
@@ -47,13 +54,13 @@ def offset
       end
 
       def per_page
-        params.fetch("per_page") do
+        params.fetch(per_page_param) do
           result_set.length
         end.to_i
       end
 
       def current_page
-        params.fetch("page", 1).to_i
+        params.fetch(page_param, 1).to_i
       end
 
       def out_of_bounds?

test/unit/top_level_links_test.rb

@@ -74,6 +74,49 @@ def test_can_parse_pagination_links
     assert_pagination
   end
 
+  def test_can_parse_pagination_links_with_custom_config
+    JsonApiClient::Paginating::Paginator.page_param = "page[number]"
+
+    stub_request(:get, "http://example.com/articles")
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: [{
+          type: "articles",
+          id: "1",
+          attributes: {
+            title: "JSON API paints my bikeshed!"
+          }
+        }],
+        links: {
+          self: "http://example.com/articles",
+          next: "http://example.com/articles?#{{page: {number: 2}}.to_query}",
+          prev: nil,
+          first: "http://example.com/articles",
+          last: "http://example.com/articles?#{{page: {number: 6}}.to_query}"
+        }
+      }.to_json)
+    stub_request(:get, "http://example.com/articles?#{{page: {number: 2}}.to_query}")
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: [{
+          type: "articles",
+          id: "2",
+          attributes: {
+            title: "This is tha BOMB"
+          }
+        }],
+        links: {
+          self: "http://example.com/articles?#{{page: {number: 2}}.to_query}",
+          next: "http://example.com/articles?#{{page: {number: 3}}.to_query}",
+          prev: "http://example.com/articles",
+          first: "http://example.com/articles",
+          last: "http://example.com/articles?#{{page: {number: 6}}.to_query}"
+        }
+      }.to_json)
+
+    assert_pagination
+
+    JsonApiClient::Paginating::Paginator.page_param = "page"
+  end
+
   def test_can_parse_pagination_links_when_no_next_page
     stub_request(:get, "http://example.com/articles")
       .to_return(headers: {content_type: "application/vnd.api+json"}, body: {