Add support for calling Hash methods on ObjectifiedHash object

Fixes #574

lib/gitlab/objectified_hash.rb

@@ -34,7 +34,14 @@ def [](key)
 
     # Respond to messages for which `self.data` has a key
     def method_missing(method_name, *args, &block)
-      data.key?(method_name.to_s) ? data[method_name.to_s] : super
+      if data.key?(method_name.to_s)
+        data[method_name.to_s]
+      elsif data.respond_to?(method_name)
+        warn 'WARNING: Please convert ObjectifiedHash object to hash before calling Hash methods on it.'
+        data.send(method_name, *args, &block)
+      else
+        super
+      end
     end
 
     def respond_to_missing?(method_name, include_private = false)

spec/gitlab/objectified_hash_spec.rb

@@ -8,6 +8,21 @@
     @oh = described_class.new @hash
   end
 
+  describe 'Hash behavior' do
+    let(:hash) { { foo: 'bar' } }
+    let(:oh) { described_class.new(hash) }
+
+    it 'allows to call Hash methods' do
+      expect(oh.dig('foo')).to eq('bar')
+      expect(oh.merge(key: :value)).to eq({ 'foo' => 'bar', key: :value })
+    end
+
+    it 'warns about calling Hash methods' do
+      output = capture_output { oh.values }
+      expect(output).to eq("WARNING: Please convert ObjectifiedHash object to hash before calling Hash methods on it.\n")
+    end
+  end
+
   it 'objectifies a hash' do
     expect(@oh.a).to eq(@hash[:a])
     expect(@oh.b).to eq(@hash[:b])