Fix bugs and add ProductPricesExport tests
	modifié:         core/lib/Thelia/Core/DependencyInjection/Loader/XmlFileLoader.php
	modifié:         core/lib/Thelia/ImportExport/Export/Type/MailingExport.php
	nouveau fichier: core/lib/Thelia/Tests/ImportExport/Export/ProductPricesExportTest.php