Merge itself with simbolized keys in rails

merge_itself_with_simbolized_keys.rb

hash = {'a' => 1, 'b' => 2}
# => {"a"=>1, "b"=>2} 

hash.merge hash.symbolize_keys
# => {"a"=>1, "b"=>2, :a=>1, :b=>2} 

hash.to_json
# => "{\"a\":1,\"b\":2}" 

hash[:a]  = 3
hash["b"] = 4

hash
# => {"a"=>1, "b"=>4, :a=>3, :b=>2}

hash.to_json
# => "{\"a\":3,\"b\":2}"