test: mock payment#externalKey in TestUtils

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: trivial cleanup in PluginHostedPaymentPageFormDescriptor

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
dao: fix serialization of currency in PluginPaymentDao

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>