Issue maxcountryman#824. Before, if a user was logged in with the login_user function when the remember parameter was set to false, their cookies would still be refreshed if the "REMEMBER_COOKIE_REFRESH_EACH_REQUEST" configuration option was set to true. This happens because if the login_user function has the remember parameter be false, it doesn't assign session["_rememeber"] any value. When session["_rememeber"] doesn't have any value and the "REMEMBER_COOKIE_REFRESH_EACH_REQUEST" configuration option is set to true, the _update_remember_cookie function sets the session["_rememeber"] value to "set". This fix makes it so if the login_user function is given false for the remember parameter, instead of leaving session["_remember"] empty, it  sets the value to "unset".

Added unit tests for each different combination of values for the "remember" and "REMEMBER_COOKIE_REFRESH_EACH_REQUEST" values.