Modify the date format to YYYY-mm-01.

Match the mozilla/ichnaea/120 changes as done by @crankycoder and send the date as YYYY-mm-01 only (overwriting the day value).
Changes as requested by @cpeterso.

Moved the code to a separate public method, controllable during execution by the relevant piece of code, depending on context.
Merge pull request #461 from tamcap/set_day_to_01

Modify the date format to YYYY-mm-01 (@tamcap)