do not use start_datetime for end_datetime when not provided

[vincentsarago]

before, when passing only one datetime we were constructing a Temporal query with date, date. But this seems to return empty response from CMR.

Fix

When only one datetime is passed we now construct a Temporal query with only one date

# before 
datetime="2018-02-12T09:00:00Z" - > {"temporal": ("2018-02-12", "2018-02-12")}

# now

datetime="2018-02-12T09:00:00Z" - > {"temporal": "2018-02-12"}