Ensure all the dates returned by the weboob modules are datetime objects. Fix #908

A user reported the issue in #908 (closed). The problem is due to the fact that dates and rdates could be date objects or datetime objects (this can varry from a module to another). To ensure it is possible to compare dates (python cannot compare date and datetime), we normalize the dates to the same class. As fromDate parameter is currently a datetime, I chose this class.