Add support for Ruby 2.5's #transform_keys(!) to IndifferentHash

lib/sinatra/indifferent_hash.rb

@@ -157,6 +157,17 @@ module Sinatra
       end
     end
 
+    if method_defined?(:transform_keys!) # Added in Ruby 2.5
+      def transform_keys(&block)
+        dup.transform_keys!(&block)
+      end
+
+      def transform_keys!
+        super
+        super(&method(:convert_key))
+      end
+    end
+
     private
 
     def convert_key(key)

test/indifferent_hash_test.rb

@@ -223,4 +223,25 @@ class TestIndifferentHash < Minitest::Test
     assert_equal :A, hash2[:a]
     assert_equal :A, hash2[?a]
   end
+
+  def test_transform_keys!
+    skip_if_lacking :transform_keys!
+
+    @hash.transform_keys! { |k| k.respond_to?(:to_sym) ? k.to_sym : k }
+
+    assert_equal :a, @hash[:a]
+    assert_equal :a, @hash[?a]
+  end
+
+  def test_transform_keys
+    skip_if_lacking :transform_keys
+
+    hash2 = @hash.transform_keys { |k| k.respond_to?(:upcase) ? k.upcase : k }
+
+    refute_equal @hash, hash2
+    refute_operator hash2, :key?, :a
+    refute_operator hash2, :key?, ?a
+    assert_equal :a, hash2[:A]
+    assert_equal :a, hash2[?A]
+  end
 end