Fixed a bug where the #total_count raises an exception

fixes #849

kaminari-core/lib/kaminari/models/page_scope_methods.rb

@@ -5,7 +5,7 @@ module Kaminari
     #   Model.page(3).per(10)
     def per(num, max_per_page: nil)
       max_per_page ||= ((defined?(@_max_per_page) && @_max_per_page) || self.max_per_page)
-      @_per = num || default_per_page
+      @_per = (num || default_per_page).to_i
       if (n = num.to_i) < 0 || !(/^\d/ =~ num.to_s)
         self
       elsif n.zero?

kaminari-core/test/models/active_record/active_record_relation_methods_test.rb

@@ -84,6 +84,10 @@ if defined? ActiveRecord
       test "throw an exception when calculating total_count when the query includes column aliases used by a group-by clause" do
         assert_equal 3, Book.joins(authorships: :user).select("users.name as author_name").group('users.name').page(1).total_count
       end
+
+      test 'total_count is calculable with page 1 per "5" (the string)' do
+        assert_equal 7, User.page(1).per('5').load.total_count
+      end
     end
   end
 end