custom_headers.rb

# this was surprisingly difficult to find, the documentation and API is poor

require "net/http"
require "uri"

url = URI.parse("http://www.whatismyip.com/automation/n09230945.asp")

req = Net::HTTP::Get.new(url.path)
req.add_field("X-Forwarded-For", "0.0.0.0")
# For content type, you could also use content_type=(type, params={})
# req.set_form_data({'query' => 'search me'})
# req['X-Forwarded-For'] = "0.0.0.0"

res = Net::HTTP.new(url.host, url.port).start do |http|
  http.request(req)
end

puts res.body