user_pages_spec.rb: added content of Hartl 9.1
users_controller.rb: added 'edit' action
users/edit.html.erb: added content of Hartl listing 9.3
spec/support/utilties.rb: added content of Hartl listing 9.6
_header.html.erb: replaced 'Settings' stub with 'edit_user_path(current_user)'
users_controller.rb: added 'update' action
user_pages_spec.rb: added content of Hartl listing 9.9
users_controller.rb: modified 'update' action to have content of Hartl listing 9.10
authentication_pages.spec: added tests in Hartl listing 9.11
users_controller.rb: added 'signed_in_user' method and before_action
users_controller.rb: fixed typo
user_pages_spec.rb: call 'sign_in user' before 'visit edit_user_path(user)'
authentication_pages_spec.rb: added tests of Hartl listing 9.14
users_controller.rb: added private methdo 'correct_user' and before_action
sessions_helper.rb: added method 'current_user?'
authentication_pages_spec.rb: added tests from Hartl listing 9.17
sessions_helper.rb: added a modified version of content from Hartl listing 9.18
users_controller.rb: modified content of 'signed_in_user' method to match that of Hartl listing 9.19
sessions_controller.rb: used 'redirect_back_or' method instead of 'redirect_to' within 'create' action
sessions_helper.rb: fixed typo: switched 'direct_to' to 'redirect_to'