#222 Label id in i18n not recognized in some cases

[Javier Paniza]

Label id in i18n not recognized if it does not exist in the i18n file corresponding at the default language of the server machine.
That is if you have in MyApp-labels_en.properties
Invoice.module=Invoices

but Invoice.module does not exist in MyApp-labels_es.properties when the server local is Spanish, then the English translation it is not recognized.
The problem is in the method exists() of Labels.