Head revision crashes on when start_time == end_time

[cpiker]

An example URL that fails to load is:

srv='https://jupiter.physics.uiowa.edu/das/server'
ds='Juno/WAV/Survey'
beg='2024-06-13'
end='2024-06-13' #<-- note same value as above
extra='LFRL'
sUrl = f'{srv}?server=dataset&dataset={ds}&start_time={beg}&end_time={end}&params={extra}'
(strm, lDs) = das2.read_http(sUrl, 3.0, "test_script")

Need to add test case for this common situation