Duplicate locals hash when setting instance vars

When a frozen locals hash is passed a RuntimeError occurs

lib/rabl/engine.rb

@@ -27,6 +27,8 @@ def source=(source)
     # Renders the representation based on source, object, context_scope and locals
     # Rabl::Engine.new("...source...", { :format => "xml" }).apply(context_scope, { :foo => "bar", :object => @user })
     def apply(context_scope, locals, &block)
+      locals = locals.dup unless locals.nil?
+
       set_instance_variables!(context_scope, locals)
 
       reset_settings!

test/engine_test.rb

@@ -23,6 +23,12 @@
     asserts_topic.assigns :_view_path
   end
 
+  context "#apply" do
+    denies("that it raises exception when given frozen locals").raises(RuntimeError) do
+      Rabl::Engine.new("").apply(Object.new, {}.freeze)
+    end
+  end
+
   context "#request_format" do
     context "is json by default" do
       setup do