Fix microseconds (#43)

* test: implement unit tests

* refactor: remove unused getClass function

* test: upgrade phpunit

* fix: retain microseconds of datetime
closes #42

Author: Greg Bowler

src/NullableTypeSafeGetter.php

@@ -26,15 +26,15 @@ public function getBool(string $name):?bool {
 	public function getDateTime(string $name):?DateTimeInterface {
 		return $this->getNullableType(
 			$name,
-			function(string|int|DateTimeInterface $value) {
+			function(string|int|float|DateTimeInterface $value) {
 				if($value instanceof DateTimeInterface) {
 					return $value;
 				}
 
 				if(is_numeric($value)) {
-					$dt = new DateTimeImmutable();
-					return $dt->setTimestamp($value);
+					return new DateTimeImmutable("@$value");
 				}
+
 				return new DateTimeImmutable($value);
 			}
 		);