@rtransform! and @rselect! aren't actually modifying dataframe

[pdeffebach]

julia> df = DataFrame(a = 1, b = 2)
1×2 DataFrame
 Row │ a      b     
     │ Int64  Int64 
─────┼──────────────
   1 │     1      2

julia> xx = ["a", "b"];

julia> for x in xx
           @rtransform! df $x = $x + 100
       end

julia> df
1×2 DataFrame
 Row │ a      b     
     │ Int64  Int64 
─────┼──────────────
   1 │     1      2