Preserve native Hash#transform_keys instead of clobbering it

lib/core_ext/stringify_keys.rb

@@ -1,12 +1,16 @@
 class Hash
-  # Stolen from ActiveSupport
-  def transform_keys
-    return enum_for(:transform_keys) { size } unless block_given?
-    result = {}
-    each_key do |key|
-      result[yield(key)] = self[key]
+  # Only define transform_keys when the native method is unavailable (Ruby < 2.5).
+  # Defining it unconditionally clobbers the native version, which also accepts a
+  # mapping hash, so downstream `transform_keys(a: :b)` calls raise ArgumentError.
+  unless method_defined?(:transform_keys)
+    def transform_keys
+      return enum_for(:transform_keys) { size } unless block_given?
+      result = {}
+      each_key do |key|
+        result[yield(key)] = self[key]
+      end
+      result
     end
-    result
   end
 
   # Returns a new hash with all keys converted to strings.

test/json_logic_test.rb

@@ -111,6 +111,15 @@ def test_filter_with_non_array
     )
   end
 
+  def test_transform_keys_mapping_hash_preserved
+    skip "native transform_keys mapping form requires Ruby >= 3.0" if RUBY_VERSION < "3.0"
+    assert_equal({ b: 1 }, { a: 1 }.transform_keys(a: :b))
+  end
+
+  def test_stringify_keys_still_works
+    assert_equal({ "a" => 1, "b" => 2 }, { a: 1, b: 2 }.stringify_keys)
+  end
+
   def test_uses_data
     assert_equal ["x", "y"], JSONLogic.uses_data(
       {