REST API - Fix Exception 

Under Windows, when retrieving an image field of a content object, I got this exception:
"Argument '$path' is invalid: Storage directory not found in image path

Fixed by replacing DIRECTORY_SEPARATOR by '/' (because path components for images in database are separated by '/' and not '\' (under windows).

Trace:
#0 D:\SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreIOHandlerLegacy.php(391): eZPublishCoreIOHandlerLegacy->removeStoragePath('var/ezsqwal/sto...')
#1 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreIOIOService.php(249): eZPublishCoreIOHandlerLegacy->getExternalPath('var/ezsqwal/sto...')
#2 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreFieldTypeImageImageStorage.php(231): eZPublishCoreIOIOService->getExternalPath('var/ezsqwal/sto...')
#3 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCorePersistenceLegacyContentStorageHandler.php(95): eZPublishCoreFieldTypeImageImageStorage->getFieldData(Object(eZPublishSPIPersistenceContentVersionInfo), Object(eZPublishSPIPersistenceContentField), Array)
#4 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCorePersistenceLegacyContentFieldHandler.php(379): eZPublishCorePersistenceLegacyContentStorageHandler->getFieldData(Object(eZPublishSPIPersistenceContentVersionInfo), Object(eZPublishSPIPersistenceContentField))
#5 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCorePersistenceLegacyContentHandler.php(323): eZPublishCorePersistenceLegacyContentFieldHandler->loadExternalFieldData(Object(eZPublishSPIPersistenceContent))
#6 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCorePersistenceCacheContentHandler.php(72): eZPublishCorePersistenceLegacyContentHandler->load(178, 2)
#7 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreRepositoryContentService.php(385): eZPublishCorePersistenceCacheContentHandler->load(178, 2, NULL)
#8 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreRepositoryContentService.php(331): eZPublishCoreRepositoryContentService->internalLoadContent(178, NULL, NULL)
#9 D:SitesSqwalEzpCanalTPEzPublishvendorezsystemsezpublish-kerneleZPublishCoreSignalSlotContentService.php(191): eZPublishCoreRepositoryContentService->loadContent(178, NULL, NULL)