[MODEL] Fixed, that param_name is used when paginating with WillPaginate

Closes elastic#307

Author: karmi

elasticsearch-model/lib/elasticsearch/model/response/pagination.rb

@@ -122,8 +122,9 @@ def length
           #     Article.search('foo').paginate(page: 1, per_page: 30)
           #
           def paginate(options)
-            page = [options[:page].to_i, 1].max
-            per_page = (options[:per_page] || klass.per_page).to_i
+            param_name = options[:param_name] || :page
+            page       = [options[param_name].to_i, 1].max
+            per_page   = (options[:per_page] || klass.per_page).to_i
 
             search.definition.update size: per_page,
                                      from: (page - 1) * per_page

elasticsearch-model/test/unit/response_pagination_will_paginate_test.rb

@@ -153,6 +153,18 @@ class WillPaginateResponse < Elasticsearch::Model::Response::Response
         assert_equal 0, @response.search.definition[:from]
         assert_equal 33, @response.search.definition[:size]
       end
+
+      should "use the param_name" do
+        @response.klass.client
+          .expects(:search)
+            .with do |definition|
+              assert_equal 10, definition[:from]
+              true
+            end
+          .returns(RESPONSE)
+
+        @response.paginate(my_page: 2, per_page: 10, param_name: :my_page).to_a
+      end
     end
 
     context "#page and #per_page shorthand methods" do