mergewith not working properly for OrderedDict

[tomyun]

julia> d1 = OrderedDict(:A => OrderedDict(:a => 1));
julia> d2 = OrderedDict(:B => OrderedDict(:b => 2));
julia> d3 = OrderedDict(:C => OrderedDict(:c => 3));
julia> d4 = OrderedDict(:D => OrderedDict(:d => 4));

With mergewith introduced in Julia 1.5 superseding merge(f::Function, ...) (JuliaLang/julia#34296), the order of OrderedDict is not maintained properly. Note that the result is in plain Dict.

julia> mergewith(merge, d1, d2, d3, d4)
Dict{Symbol, OrderedDict{Symbol, Int64}} with 4 entries:
  :A => OrderedDict(:a=>1)
  :D => OrderedDict(:d=>4)
  :B => OrderedDict(:b=>2)
  :C => OrderedDict(:c=>3)

Compared to the result from merge which maintains type/order correctly as the behavior is explicitly defined for OrderedDict.

julia> merge(merge, d1, d2, d3, d4)
OrderedDict{Symbol, OrderedDict{Symbol, Int64}} with 4 entries:
  :A => OrderedDict(:a=>1)
  :B => OrderedDict(:b=>2)
  :C => OrderedDict(:c=>3)
  :D => OrderedDict(:d=>4)

Defining mergewith (and perhaps mergewith! too) for OrderedDict in the same way as the custom merge seems to make it working.

https://github.com/JuliaCollections/OrderedCollections.jl/blob/844098f75f13375642b9d2bf4efdf08c691403a7/src/ordered_dict.jl#L471-L474

julia> OrderedCollections.mergewith(combine, d::OrderedDict, others::AbstractDict...) = begin
           K,V = OrderedCollections._merge_kvtypes(d, others...)
           mergewith!(combine, OrderedDict{K,V}(), d, others...)
       end

julia> mergewith(merge, d1, d2, d3, d4)
OrderedDict{Symbol, OrderedDict{Symbol, Int64}} with 4 entries:
  :A => OrderedDict(:a=>1)
  :B => OrderedDict(:b=>2)
  :C => OrderedDict(:c=>3)
  :D => OrderedDict(:d=>4)

Tested with OrderedCollections 1.4.0 on Julia 1.6.