Modify return value of String#slice! if idx == self.size

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -169,7 +169,7 @@ class String
     if arg1 != nil && arg2 != nil
       idx = arg1
       idx += self.size if arg1 < 0
-      if idx >= 0 && idx < self.size && arg2 > 0
+      if idx >= 0 && idx <= self.size && arg2 > 0
         str = self[idx, arg2]
       else
         return nil

mrbgems/mruby-string-ext/test/string.rb

@@ -218,7 +218,7 @@ assert('String#slice!') do
   assert_equal "Foo", a
 
   a = "FooBar"
-  assert_nil a.slice!(6, 2)
+  assert_equal "", a.slice!(6, 2)
   assert_equal "FooBar", a
 
   a = "FooBar"