allow override of default PaginatedCollection options.

reverse order of _.extend call in PaginatedCollection to allow
individual collections to override the add and url options.

Change-Id: Ia49e6a549b11023304fbee9e3465cbec3694b48f
Reviewed-on: https://gerrit.instructure.com/12676
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>

Author: zachpendleton

app/coffeescripts/collections/PaginatedCollection.coffee

@@ -7,9 +7,7 @@ define [
 
     fetchNextPage: (options={}) ->
       throw new Error "can't fetch next page when @nextPageUrl is undefined" unless @nextPageUrl?
-      options = _.extend {}, options,
-        add: true
-        url: @nextPageUrl
+      options = _.extend({}, {add: true, url: @nextPageUrl}, options)
 
       @fetchingNextPage = true
       @trigger 'beforeFetchNextPage'
@@ -51,4 +49,4 @@ define [
 
     parse: (response, xhr) ->
       @_parsePageLinks(xhr)
-      super
+      super