copy does not copy to standard Julia Types

While the documentation says that a copy would ensure to have normal Julia Types

> df = copy(DataFrame(Arrow.Table(file))): Build a [DataFrame](https://juliadata.github.io/DataFrames.jl/stable/), where the columns are regular in-memory vectors (specifically, Base.Vectors and/or PooledVectors). This requires that you have enough memory to load the entire DataFrame into memory.

this is not the case


Start by reproducing `copy(DataFrame(Arrow.Table(file)))` and inspect the resulting column types against the documentation quoted in the issue. Trace the copy behavior from that entry point; done means the observed behavior and the documented expectation agree, with a regression check for the reported case.

Written by the indexing model from the issue text.