Fix some time issues

[ldecicco-USGS]

Should now allow this:

start <- "2025-10-01"
> end <- Sys.Date()
> dataRetrieval:::format_api_dates(c(start, end), date = TRUE)
[1] "2025-10-01/2026-02-02"

(which fixes #853 )

and this:

dataRetrieval:::format_api_dates(c("2010-01-01T00:00Z", "2010-01-01T05:00Z"))
[1] "2010-01-01T00:00:00Z/2010-01-01T05:00:00Z"

(which fixes #839 )