Fixed PIM-3244 memory leak on products with media export

TODO : 
 - [x] update specs for CsvProductWriter
 - [x] update specs for MediaNormalizer

Expected format
array(2) {
  [0] =>
  array(2) {
    'filePath' =>
    string(101) "/home/nico/git/pim-ce-orm/app/uploads/product/75-aknts_bpxs_vso-picture---1412677666-notification.gif"
    'exportPath' =>
    string(45) "files/AKNTS_BPXS_VSO/picture/notification.gif"
  }
  [1] =>
  array(2) {
    'filePath' =>
    string(99) "/home/nico/git/pim-ce-orm/app/uploads/product/75-aknts_bpxs_vso-testmedia---1412688276-8rymncgx.jpg"
    'exportPath' =>
    string(43) "files/AKNTS_BPXS_VSO/testmedia/8rymncgx.jpg"
  }
}

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | ?
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3244
| Doc PR               |