InMemoryUserDetailsManager.changePassword fails for non-lowercase usernames

[junhyeong9812]

Describe the bug

InMemoryUserDetailsManager.changePassword(...) cannot change the password of a user whose username contains uppercase letters.

The manager keys its internal user map on the lower-cased username in every other method (createUser, updateUser, deleteUser, userExists, loadUserByUsername, updatePassword), but changePassword(...) looks the current user up with the raw Authentication.getName().

To Reproduce

InMemoryUserDetailsManager manager = new InMemoryUserDetailsManager(
        User.withUsername("User").password("{noop}password").roles("USER").build());

// authenticate as "User" (e.g. place a UsernamePasswordAuthenticationToken("User", ...) in the SecurityContext)

manager.changePassword("password", "newPassword");
// throws IllegalStateException: Current user doesn't exist in database.

The user is stored under the key "user", but changePassword calls users.get("User"), which returns null.

Expected behavior

The password is changed, consistent with every other method on the manager, which treat the username case-insensitively.

I will submit a PR with a regression test.