`invalid index` with SubDataFrame and `copycols = false`

[jkrumbiegel]

MWE:

julia> df = DataFrame(a = [1, 2, 3])
3×1 DataFrame
 Row │ a     
     │ Int64 
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> v = @view df[1:2, :]
2×1 SubDataFrame
 Row │ a     
     │ Int64 
─────┼───────
   1 │     1
   2 │     2

julia> select(v, :a => ByRow(x -> x + 1) => :a)
2×1 DataFrame
 Row │ a     
     │ Int64 
─────┼───────
   1 │     2
   2 │     3

julia> select(v, :a => ByRow(x -> x + 1) => :a; copycols = false)
ERROR: ArgumentError: invalid index: :a => (ByRow{var"#618#619"}(var"#618#619"()) => :a) of type Pair{Symbol, Pair{ByRow{var"#618#619"}, Symbol}}
Stacktrace:
 [1] getindex(x::DataFrames.Index, idx::Pair{Symbol, Pair{ByRow{var"#618#619"}, Symbol}})
   @ DataFrames ~/.julia/packages/DataFrames/rwKZk/src/other/index.jl:183
 [2] manipulate(dfv::SubDataFrame{…}, args::Any; copycols::Bool, keeprows::Bool, renamecols::Bool)
   @ DataFrames ~/.julia/packages/DataFrames/rwKZk/src/abstractdataframe/selection.jl:1836
 [3] select(df::SubDataFrame{…}, args::Any; copycols::Bool, renamecols::Bool, threads::Bool)
   @ DataFrames ~/.julia/packages/DataFrames/rwKZk/src/abstractdataframe/selection.jl:1303
 [4] top-level scope
   @ REPL[25]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> select(df, :a => ByRow(x -> x + 1) => :a; copycols = false)
3×1 DataFrame
 Row │ a     
     │ Int64 
─────┼───────
   1 │     2
   2 │     3
   3 │     4