Allow access of aliases as symbols or strings just like its param

Prior to this rev, parameter aliases can only be access via dot
notation. This change allows one to use any of the following:

  keystone.request(:create_token) do |p|
    p[:tenant_name] = 'name'
  end

  keystone.request(:create_token) do |p|
    p['tenant_name'] = 'name'
  end

  keystone.request(:create_token) do |p|
    p.tenant_name = 'name'
  end