Estoy usando la API HTTP de Mailgun para enviar correos electrónicos desde la aplicación de rieles, pero no puedo mostrar las vistas de correo. Esta es mi clase de correo.

class MailgunMails < ActionMailer::Base

    def send_complex_message
      data = Multimap.new
      data[:from] = "Excited User <postmasters@#{@@domain}>"
      data[:to] = "[email protected]"
      data[:subject] = "Hello"
      data[:html] = File.open("#{Rails.root}/app/views/mailgun_mails/send_complex_message.html.erb", 'r').to_s.html_safe  
      ## also tried
      data[:html] = render(template: "send_complex_message.html.erb")
      response = RestClient.post "https://api:#{@@private_key}"\
      "@api.mailgun.net/v2/#{@@domain}/messages", data

      response = JSON.parse(response)

    end

end  

Pero eso arroja:

NoMethodError: método indefinido `each_byte 'para nil: NilClass

¿Cómo uso las vistas de correo con la API Mailgun?

PD: Sé que hay muchas gemas disponibles para usar mailgun con rieles, pero ninguna de ellas ofrece una funcionalidad completa como lo hace la API.

SI hay alguna forma alternativa de enviar correos electrónicos desde la API usando las vistas de rieles, estoy esperando esa respuesta. Simplemente no sé cómo hacerlo (incluso si no usa actionmailer)

Seguimiento de error completo

NoMethodError: undefined method `each_byte' for nil:NilClass
    from /Users/example/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/uri/common.rb:307:in `block in escape'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/activesupport-4.0.4/lib/active_support/core_ext/string/output_safety.rb:177:in `gsub'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/activesupport-4.0.4/lib/active_support/core_ext/string/output_safety.rb:177:in `gsub'
    from /Users/example/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/uri/common.rb:304:in `escape'
    from /Users/example/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/uri/common.rb:623:in `escape'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:150:in `handle_key'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:143:in `block in build_stream'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:142:in `collect'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:142:in `build_stream'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:53:in `initialize'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:18:in `new'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/payload.rb:18:in `generate'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/request.rb:45:in `initialize'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/request.rb:33:in `new'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient/request.rb:33:in `execute'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/rest-client-1.6.7/lib/restclient.rb:72:in `post'
... 2 levels...
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionpack-4.0.4/lib/abstract_controller/callbacks.rb:18:in `block in process_action'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/activesupport-4.0.4/lib/active_support/callbacks.rb:373:in `_run__1281843520849433682__process_action__callbacks'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/activesupport-4.0.4/lib/active_support/callbacks.rb:80:in `run_callbacks'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionpack-4.0.4/lib/abstract_controller/callbacks.rb:17:in `process_action'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionpack-4.0.4/lib/abstract_controller/base.rb:136:in `process'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionpack-4.0.4/lib/abstract_controller/rendering.rb:44:in `process'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionmailer-4.0.4/lib/action_mailer/base.rb:503:in `process'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionmailer-4.0.4/lib/action_mailer/base.rb:497:in `initialize'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionmailer-4.0.4/lib/action_mailer/base.rb:480:in `new'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/actionmailer-4.0.4/lib/action_mailer/base.rb:480:in `method_missing'
    from (irb):1
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/railties-4.0.4/lib/rails/commands/console.rb:90:in `start'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/railties-4.0.4/lib/rails/commands/console.rb:9:in `start'
    from /Users/example/.rvm/gems/ruby-2.0.0-p451/gems/railties-4.0.4/lib/rails/commands.rb:62:in `<top (required)>'
    from bin/rails:4:in `require'