Do not promote to Any

[tkf]

Before:

julia> push!!([Init(+)], 0)
2-element Array{Any,1}:
  Init(+)
 0

After:

julia> push!!([Init(+)], 0)
2-element Array{Union{InitialValues.InitialValueOf{typeof(+)}, Int64},1}:
  Init(+)
 0

[codecov-io]

Codecov Report

Merging #27 into master will decrease coverage by 1.83%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   91.83%   90.00%   -1.84%     
==========================================
  Files           2        2              
  Lines          49       50       +1     
==========================================
  Hits           45       45              
- Misses          4        5       +1     

Impacted Files	Coverage Δ
src/InitialValues.jl	85.71% <0.00%> (-2.53%)	⬇️

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