How do I make Ruby's RestClient gem respect content_type on post?

For instance, in the RestClient console:

RestClient.post 'http://localhost:5001', {:a => 'b'}, :content_type => 'application/json'

This does not send application/json as the content type. Instead I see:

Content-Type: application/x-www-form-urlencoded

I was able to trace the change to restclient/payload.rb:

  class UrlEncoded < Base
  ...

  def headers
    super.merge({'Content-Type' => 'application/x-www-form-urlencoded'})
  end
end

Replacing super.merge with super causes the content type to be respected, but obviously that's not a real solution. Does anyone know the proper way to fix this? Thanks.