class Restforce::Middleware::Authentication::Password

def authenticate!
  response = connection.post '/services/oauth2/token' do |req|
    req.body = URI.encode_www_form(
      :grant_type    => 'password',
      :client_id     => @options[:client_id],
      :client_secret => @options[:client_secret],
      :username      => @options[:username],
      :password      => password
    )
  end
  raise Restforce::AuthenticationError if response.status != 200
  @options[:instance_url] = response.body['instance_url']
  @options[:oauth_token]  = response.body['access_token']
end