replace as.character with format for R4.3

[klin333]

in R4.3 the following could give warning:

rhandsontable::rhandsontable(data.frame(x = Sys.Date()))
# Warning message:
# In as.character.POSIXt(as.POSIXlt(x), ...) :
#   as.character(td, ..) no longer obeys a 'format' argument; use format(td, ..) ?

This pull request fixes it

[petersmp]

I am also running into this issue. It looks like this PR would fix the issue.

[J-flem]

Thank you for this branch @klin333. Hopefully it can be pulled into the main branch soon