Maintain subclasses of Hash when calling Hash#slice.

Fixes #250

lib/i18n/core_ext/hash.rb

@@ -1,6 +1,6 @@
 class Hash
   def slice(*keep_keys)
-    h = {}
+    h = self.class.new
     keep_keys.each { |key| h[key] = fetch(key) if has_key?(key) }
     h
   end unless Hash.method_defined?(:slice)
@@ -21,7 +21,7 @@ def deep_symbolize_keys
   MERGER = proc do |key, v1, v2|
     Hash === v1 && Hash === v2 ? v1.merge(v2, &MERGER) : v2
   end
-  
+
   def deep_merge!(data)
     merge!(data, &MERGER)
   end unless Hash.method_defined?(:deep_merge!)

test/core_ext/hash_test.rb

@@ -20,6 +20,12 @@ class I18nCoreExtHashInterpolationTest < I18n::TestCase
     assert_equal expected, hash.slice(:foo, :not_here)
   end
 
+  test "#slice maintains subclasses of Hash" do
+    klass = Class.new(Hash)
+    hash = klass[:foo, 'bar', :baz, 'bar']
+    assert_instance_of klass,  hash.slice(:foo)
+  end
+
   test "#except" do
     hash = { :foo => 'bar',  :baz => 'bar' }
     expected = { :foo => 'bar' }