devise.rb

#config/initializers/devise.rb

  config.warden do |manager|
    manager.strategies.add :token_header_authenticatable, TokenHeaderAuthenticatable
    manager.default_strategies(:scope => :user).unshift :token_header_authenticatable
  end

token_header_authenticatable.rb

#lib/token_header_authenticable.rb

class TokenHeaderAuthenticatable < ::Devise::Strategies::Base
  def valid?
    token_value.present?
  end

  def authenticate!
    resource = mapping.to.find_for_token_authentication(auth_token: token_value)

    if resource
      success!(resource)
    else
      fail!
    end
  end

  private

  def token_value
    if header && header =~ /^Token token="(.+)"$/
      $~[1]
    end
  end

  def header
    request.headers["Authorization"]
  end
end

The changes were to rename things to be consistent with Devise and simplify the authenticate method.