When dealing with `Optional` values in a Controller handler (for
example, values coming from a Spring Data repository), developers might
reuse this code snippet quite often:
```
@GetMapping("/user")
public ResponseEntity<Optional<User>> fetchUser() {
Optional<User> user = //...
return user.map(ResponseEntity::ok).orElse(notFound().build());
}
```
This commit adds a new static method on `ResponseEntity` for that,
simplifying the previous snippet with `return ResponseEntity.of(user);`
Note that in case more specific HTTP response headers are required by
the application, developers should use other static methods to
explicitly tell which headers should be used in each case.
Issue: SPR-17187
432cdd7802