`transform_vec` doesn't replace existing column

[xiaodaigh]

See MWE:

t = JuliaDB.table((a = [1,2, missing, 4, missing],))
t = @transform_vec t {a = coalesce(:a, 2)}

a is not replaced, but the below is fine, the new column b is created automatically.

t = JuliaDB.table((a = [1,2, missing, 4, missing],))
t = @transform_vec t {b = coalesce(:a, 2)}