(Closes issue #131) Make cal_min_max_date (and by extension oak_cal_ref_dates) work with Date objects and hms time objects (not just character date and time strings)

[zhengzheli]

#131
Create_iso8601 in cal_min_max_date expects a character string, but cal_min_max_date may refer to date variables and time variables that are not in a string format.
For example, the date may be a Date object, and the time may be an hms object, if the haven::read_sas() function was used to convert a sas dataset into an R dataframe.
So we make a small adjustment to allow for Date objects and hms time objects.

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