Change to typesubtract signature causes failure with Julia 1.6-dev

[andreasnoack]

julia> CSV.File("/Users/andreasnoack/Downloads/data.csv")
ERROR: MethodError: no method matching typesubtract(::Type{Union{}}, ::Type{Int64})
Closest candidates are:
  typesubtract(::Any, ::Any, ::Int64) at compiler/typeutils.jl:66
Stacktrace:
 [1] ts(T::Type, S::Type)
   @ CSV ~/.julia/packages/CSV/MKemC/src/utils.jl:214
 [2] nonstandardtype(T::Type)
   @ CSV ~/.julia/packages/CSV/MKemC/src/utils.jl:217
 [3] (::CSV.var"#17#23")(T::Type)
   @ CSV ./none:0
 [4] iterate(::Base.Iterators.Filter{CSV.var"#17#23", Vector{Type}})
   @ Base.Iterators ./iterators.jl:451
 [5] iterate
   @ ./generator.jl:44 [inlined]
 [6] Header
   @ ~/.julia/packages/CSV/MKemC/src/header.jl:232 [inlined]
 [7] CSV.File(source::String; header::Int64, normalizenames::Bool, datarow::Int64, skipto::Nothing, footerskip::Int64, transpose::Bool, comment::Nothing, use_mmap::Nothing, ignoreemptylines::Bool, select::Nothing, drop::Nothing, missingstrings::Vector{String}, missingstring::String, delim::Nothing, ignorerepeated::Bool, quotechar::Char, openquotechar::Nothing, closequotechar::Nothing, escapechar::Char, dateformat::Nothing, dateformats::Nothing, decimal::UInt8, truestrings::Vector{String}, falsestrings::Vector{String}, type::Nothing, types::Nothing, typemap::Dict{Type, Type}, categorical::Nothing, pool::Float64, lazystrings::Bool, strict::Bool, silencewarnings::Bool, debug::Bool, parsingdebug::Bool, kw::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{, Tuple{}}})
   @ CSV ~/.julia/packages/CSV/MKemC/src/file.jl:216
 [8] CSV.File(source::String)
   @ CSV ~/.julia/packages/CSV/MKemC/src/file.jl:216
 [9] top-level scope
   @ REPL[39]:1

[quinnj]

This is fixed on master

[timholy]

With Julia 1.6 entering stabilization phase shortly, would be great to get a new release if the code is in a shape where that's fairly straightforward.

[quinnj]

Ok, we have the 0.7.8 release and 0.8 release that both include the fix