Unwarranted, panic-inducing error message in Series.bin()

[CommonClimate]

Describe the bug
When specifying a bin_size argument in ts.bin(), one gets the message:
UserWarning: The step, time axis, or bin edges have been set, the series may not be evenly_spaced

To Reproduce

ts = pyleo.utils.load_dataset('EDC-dD')
ts.bin(bin_size=5).plot()

Expected behavior
The warning should appear only if no_nans = True and is_evenly_spaced = False. Also, the behavior of gkernel should be identical (verify that interp doesn't need such treament)

[CommonClimate]

addressed by #399