Resume Recommendations Request/Response

This change is to add resume recommendations to the external api wrapper.
In order to test, copy and paste the code below into terminal (replacing dev key with real one)..........

bundle
bundle exec pry
require 'Cb'

Cb.configure do |config|
  config.dev_key = 'place_dev_key_here'
  config.time_out = 30
  config.debug_api = true
end

client = Cb::Client.new
request = Cb::Requests::Recommendations::Resume.new({:resume_hash => '9b13d2082bfeb71d36d109ae300f066adaea64e6770ec055252f1c0dfb3eee09', :external_user_id => 'XRHN41N711FXMCP6CVQD'})

client.execute(request)
I'm with you.  nil.to_s seems strange.  The reason I did it this way was to match what I saw in resume_get and application_get.  I'm down to change it if the team feels there is a better solution.  If I change it here I would like to change it everywhere to keep things consistent.  Let me know what yall think.