Roundtrippability of special strings

[nilshg]

As discussed on Slack, it would be nice if this would work:

julia> using Arrow, DataFrames, ShortStrings

julia> df = DataFrame(stringcol = ShortString7.(["abcde", "fghij"]))
2×1 DataFrame
 Row │ stringcol 
     │ ShortStr… 
─────┼───────────
   1 │ abcde
   2 │ fghij

julia> Arrow.write("test.arrow", df);

julia> DataFrame(Arrow.Table("test.arrow"))
2×1 DataFrame
 Row │ stringcol 
     │ String    
─────┼───────────
   1 │ abcde
   2 │ fghij