Fiksing problem with memento-headers on machines with non-english locales

Building OpenWayback on machines with non-english locale fails on one unit-test. The test is expecting a date-header in english locale, but MementoUtils does not set the locale on the SimpleDateFormat and the machines locale is used.

This patch fixes the locale problem and sets to default locale to french for the test to make sure that the test will act the same independent on locale settings on the build-machine.