mruby-hash-ext/hash.rb: make transform_keys! to keep old keys

```ruby
h={1=>2,2=>3}
h.transform_keys!{_1.succ}
```

used to make `h` to `{2=>2}` (duplicated keys were squashed), but now
makes it `{2=>2,3=>3}`.

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

@@ -428,12 +428,8 @@ class Hash
   #
   def transform_keys!(&block)
     return to_enum :transform_keys! unless block
-    self.keys.each do |k|
-      value = self[k]
-      self.__delete(k)
-      k = block.call(k) if block
-      self[k] = value
-    end
+    hash = self.transform_keys(&block)
+    self.replace(hash)
     self
   end
   ##