mystrategy.rb

require 'devise/strategies/base'

module Devise
  module Strategies
    # Sign in an user using HTTP authentication.
    class HttpAuthenticatableWithContentType < Base
      MESSAGE = "Could not authenticate you."
      
      def valid?
        request.authorization && mapping.to.respond_to?(:authenticate_with_http)
      end

      def authenticate!
        username, password = username_and_password

        if resource = mapping.to.authenticate_with_http(username, password)
          success!(resource)
        else
          custom!([401, custom_headers, custom_message])
        end
      end

    private

      def username_and_password
        decode_credentials(request).split(/:/, 2)
      end

      def decode_credentials(request)
        ActiveSupport::Base64.decode64(request.authorization.split(' ', 2).last || '')
      end

      def custom_headers
        {
          "Content-Type" => request.format.to_s,
          "WWW-Authenticate" => %(Basic realm="#{Devise.http_authentication_realm.gsub(/"/, "")}")
        }
      end
      
      def custom_message
        case request.format.to_s
          when 'application/xml':  [ { :error => MESSAGE }.to_xml ]
          when 'application/json': [ { :error => MESSAGE }.to_json ]
          else
            [ MESSAGE ]
        end
      end
    end
  end
end

Warden::Strategies.add(:http_authenticatable_with_content_type, Devise::Strategies::HttpAuthenticatableWithContentType)