Let Hash#slice return a Hash

In order to keep this method compatible with the Ruby 2.5 version of Hash#slice.
This bahavior is actually slightly incompatibile with previous versions of Active Support
but it might not cause a real problem, since HWIA, the biggest use case of Hash subclassing here,
already overrides `slice` to return another HWIA.

activesupport/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   `Hash#slice` now falls back to Ruby 2.5+'s built-in definition if defined.
+
+    *Akira Matsuda*
+
 *   Deprecate `secrets.secret_token`.
 
     The architecture for secrets had a big upgrade between Rails 3 and Rails 4,

activesupport/lib/active_support/core_ext/hash/slice.rb

@@ -21,7 +21,7 @@ class Hash
   #   valid_keys = [:mass, :velocity, :time]
   #   search(options.slice(*valid_keys))
   def slice(*keys)
-    keys.each_with_object(self.class.new) { |k, hash| hash[k] = self[k] if has_key?(k) }
+    keys.each_with_object(Hash.new) { |k, hash| hash[k] = self[k] if has_key?(k) }
   end unless method_defined?(:slice)
 
   # Replaces the hash with only the given keys.