Frame.copy() does not copy key columns #1443
Description
This fails:
>>> import datatable as dt
>>> DT = dt.Frame(A=[1,2,3], B=['f', 'wer', 'd'])
>>> DT.key = "A"
>>> DT2 = DT.copy()
>>> assert DT2.key == DT.key
{{ message }}
This fails:
>>> import datatable as dt
>>> DT = dt.Frame(A=[1,2,3], B=['f', 'wer', 'd'])
>>> DT.key = "A"
>>> DT2 = DT.copy()
>>> assert DT2.key == DT.key