class Restforce::Middleware::Authentication

def authenticate!
  response = connection.post '/services/oauth2/token' do |req|
    req.body = encode_www_form(params)
  end
  if response.status >= 500
    raise Restforce::ServerError, error_message(response)
  elsif response.status != 200
    raise Restforce::AuthenticationError, error_message(response)
  end
  @options[:instance_url] = response.body['instance_url']
  @options[:oauth_token]  = response.body['access_token']
  if @options[:authentication_callback]
    @options[:authentication_callback].call(response.body)
  end
  response.body
end