Merge on DefaultOrderedDicts should call the ordered merge function

[tlnagy]

julia> d = DefaultOrderedDict{String, Vector{UInt16}}(Vector{UInt16});

julia> d2 = DefaultOrderedDict{String, Vector{UInt16}}(Vector{UInt16});

julia> @which merge(d, d2)
merge(d::AbstractDict, others::AbstractDict...) in Base at abstractdict.jl:311

julia> @which merge(OrderedDict(d), OrderedDict(d2))
merge(d::OrderedDict, others::AbstractDict...) in OrderedCollections at /home/tlnagy/.julia/packages/OrderedCollections/PRayh/src/ordered_dict.jl:466

Because the regular dict merge function is called the order is lost when merging two DefaultOrderedDicts.