Handle missing better in `DefaultDict`

Modified from https://stackoverflow.com/questions/57996377/create-array-of-values-based-on-dictionary-and-array-of-keys:

Say we have a dictionary, and a vector of keys:

X = [2, 1, 1, missing]
d = Dict( 1 => "A", 2 => "B", 3 => "C")

I want to create a new array which contains values instead of keys (according to the dictionary), so the end result would be something like

Y = ["B", "A", "A", "C"]

The basic solution is to modify d and use a comprehension:

d = Dict( 1 => "A", 2 => "B", 3 => "C", missing => missing)
Y = [d[i] for i in X]

I thought I could create a DefaultDict that would handle this slightly nicer, but the end result was less than satisfying:

julia> d = DefaultDict{Union{Int,Missing},Union{String,Missing},Union{String,Missing}}(missing, 1 => "A", 2 => "B", 3 => "C")
ERROR: MethodError: no method matching DefaultDict{Union{Missing, Int64},Union{Missing, String},Union{Missing, String}}(::Missing, ::Pair{Int64,String}, ::Pair{Int64,String}, ::Pair{Int64,String})
Closest candidates are:
  DefaultDict{Union{Missing, Int64},Union{Missing, String},Union{Missing, String}}(::Any, ::Pair{K,V}...; kwargs...) where {K, V, F} at /Users/kevinsquire/.julia/dev/DataStructures/src/default_dict.jl:98
  DefaultDict{Union{Missing, Int64},Union{Missing, String},Union{Missing, String}}(::Any; kwargs...) where {K, V, F} at /Users/kevinsquire/.julia/dev/DataStructures/src/default_dict.jl:105
  DefaultDict{Union{Missing, Int64},Union{Missing, String},Union{Missing, String}}(::Any, ::AbstractArray{Tuple{K,V},N} where N; kwargs...) where {K, V, F} at /Users/kevinsquire/.julia/dev/DataStructures/src/default_dict.jl:100
  ...
Stacktrace:
 [1] top-level scope at REPL[16]:1

julia> d = DefaultDict{Union{Int,Missing},Union{String,Missing},Union{String,Missing}}(missing);

julia> merge!(d, Dict(1 => "A", 2 => "B", 3 => "C"));

julia> Y = [d[i] for i in X]
4-element Array{Union{Missing, String},1}:
 "B"
 "A"
 "A"
 missing

It would be nice to deal with this better, either by supporting missing/Missing in DefaultDict, or in some other way that makes it easy to handle missingness in dictionaries.

Reproduce the shown constructor error and inspect the DefaultDict constructors in src/default_dict.jl, especially the methods around line 98. Compare the constructor behavior with the merge! and comprehension examples, then determine and document a supported missing/Missing behavior; done means the demonstrated lookup workflow handles missing keys without the reported error.