always call jsonapi_page_size method on pagination enabled requests (#40)

* always call jsonapi_page_size method on pagination enabled requests

pass the per_page_param to the jsonapi_page_size method

* pass pagination to jsonapi_page_size and handle string-to-int conversion in jsonapi_page_size

* update readme to reflect changes

* fix woring

Author: fluxsaas

README.md

@@ -308,12 +308,14 @@ use the `jsonapi_pagination_meta` method:
 
 ```
 
-If you want to change the default number of items per page, use the
+If you want to change the default number of items per page or define a custom logic to handle page size, use the
 `jsonapi_page_size` method:
 
 ```ruby
-  def jsonapi_page_size
-    30
+  def jsonapi_page_size(pagination_params)
+    per_page = pagination_params[:size].to_f.to_i
+    per_page = 30 if per_page > 30
+    per_page
   end
 ```
 ### Deserialization

lib/jsonapi/pagination.rb

@@ -96,18 +96,27 @@ def jsonapi_pagination_meta(resources)
     # @return [Array] with the offset, limit and the current page number
     def jsonapi_pagination_params
       pagination = params[:page].try(:slice, :number, :size) || {}
-      per_page = pagination[:size].to_f.to_i
-      per_page = jsonapi_page_size if per_page < 1
+      per_page = jsonapi_page_size(pagination)
       num = [1, pagination[:number].to_f.to_i].max
 
       [(num - 1) * per_page, per_page, num]
     end
 
     # Retrieves the default page size
     #
+    # @param per_page_param [Hash] opts the paginations params
+    # @option opts [String] :number the page number requested
+    # @option opts [String] :size the page size requested
+    #
     # @return [Integer]
-    def jsonapi_page_size
-      self.class.const_get(:JSONAPI_PAGE_SIZE).to_i
+    def jsonapi_page_size(pagination_params)
+      per_page = pagination_params[:size].to_f.to_i
+
+      return self.class
+              .const_get(:JSONAPI_PAGE_SIZE)
+              .to_i if per_page < 1
+
+      per_page
     end
 
     # Fallback to Rack's parsed query string when Rails is not available