Add a guard for Proc#ruby2_keywords

Author: mame

lib/ruby2_keywords.rb

@@ -15,7 +15,9 @@ def main.ruby2_keywords(name, *)
 end
 
 class Proc
-  def ruby2_keywords
-    self
+  unless method_defined?(:ruby2_keywords)
+    def ruby2_keywords
+      self
+    end
   end
 end