Checking in multiple times doesn't raise an error

The API calls being done over mobile network it could happen that the
client doesn't get any feedback after a check-in, and would then
check-in a second time, in that case it should just confirm that the
rental is no longer ongoing. If there is no ongoing rental then it's
fine. Also the response will receive an empty response confirming that
the rental was already checked-in
Merge pull request #74 from DonkeyRepublik/bastien/fix-double-checkin

Checking in multiple times doesn't raise an error