Added boolean check before setting the date

DateTime::createFromFormat(...) returns either a \DateTime instance or FALSE, not NULL.  Working on the basis that there might be legitimate TrackChange constructor calls containing a NULL date parameter, line 67 has been expanded to check not only for NOT NULL but also NOT FALSE.

Without this additional check, a document with "Track Changes" enabled could cause an Exception to be raised when loaded into a reader.

"Exception: DateTime::__construct(): Failed to parse time string (@) at position 0 (@): Unexpected character"

Author: superhaggis

src/PhpWord/Element/TrackChange.php

@@ -58,13 +58,13 @@ class TrackChange extends AbstractContainer
      *
      * @param string $changeType
      * @param string $author
-     * @param null|int|\DateTime $date
+     * @param null|int|bool|\DateTime $date
      */
     public function __construct($changeType = null, $author = null, $date = null)
     {
         $this->changeType = $changeType;
         $this->author = $author;
-        if ($date !== null) {
+        if ($date !== null && $date !== false) {
             $this->date = ($date instanceof \DateTime) ? $date : new \DateTime('@' . $date);
         }
     }