server: json.Unmarshal fails after every test

[robertodauria]

The latest release (v0.2.0) of dash-server fails after every test with the following error:

35.204.253.208 - - [12/Sep/2019:13:05:08 +0000] "GET /dash/download/37253500 HTTP/1.1" 200 7500000
35.204.253.208 - - [12/Sep/2019:13:05:08 +0000] "GET /dash/download/37289750 HTTP/1.1" 200 7500000
{"fields":{},"level":"warn","timestamp":"2019-09-12T13:05:08.98904272Z","message":"collect: json.Unmarshal: json: cannot unmarshal number 0.0 into Go struct field ClientResults.delta_sys_time of type int64"}
35.204.253.208 - - [12/Sep/2019:13:05:08 +0000] "POST /collect/dash HTTP/1.1" 400 0

This causes test results to not be written to the output folder.