DataCollector accumulated date bugs

• The DataCollector sets for example Response.accumulated_network_usage.sent_b_wo_admin.rate to 0 instead of None while there is no old enough data to calculate the rate. Therefore (because the DataCollector provides wrong values) the gui displays 0 MB/s as rate, which shouldn't be the case.
• The Responses Response.accumulated_network_usage attribute is None in around 50% of requests, although accumulated_network_usage is continuously requested (not only on startup, but the whole time).