Don't accept negative value for padding

Because negative offset behavior is so unpredictable: https://github.com/amatsuda/kaminari/issues/839#issuecomment-268211318

closes #839

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

@@ -25,7 +25,9 @@ module Kaminari
     end
 
     def padding(num)
-      @_padding = num.to_i
+      num = num.to_i
+      raise ArgumentError, "padding must not be negative" if num < 0
+      @_padding = num
       offset(offset_value + @_padding)
     end
 

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

@@ -203,6 +203,10 @@ if defined? ActiveRecord
             assert_equal 19, relation.current_page
             assert_equal 19, relation.total_pages
           end
+
+          test 'Negative padding' do
+            assert_raise(ArgumentError) { model_class.page(1).per(5).padding(-1) }
+          end
         end
 
         sub_test_case '#total_pages' do

kaminari-core/test/models/array_test.rb

@@ -88,6 +88,10 @@ class PaginatableArrayTest < ActiveSupport::TestCase
     test 'per 25, padding 25' do
       assert_equal 3, @array.page(1).padding(25).total_pages
     end
+
+    test 'Negative padding' do
+      assert_raise(ArgumentError) { @array.page(1).per(5).padding(-1) }
+    end
   end
 
   sub_test_case '#total_pages' do