Fix createdAt date range filter

- hiddenName has been disabled in order to fix the missing date range
  values when using the material theme
- data format has been changed to 'Y-m-d' in order to comply with the
  browser date input default format
- tests: date() and strtotime have been replaced with DateTime-related
  objects

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Wallabag/CoreBundle/Entity/Entry.php

@@ -552,7 +552,7 @@ class Entry
      *
      * @return Entry
      */
-    public function setCreatedAt(\DateTime $createdAt)
+    public function setCreatedAt(\DateTimeInterface $createdAt)
     {
         $this->createdAt = $createdAt;