Fix show for const properties of type Vector or Symbol

[SteffenPL]

It seems that

@proto mutable struct Foo
    const x::Vector{Int} = Int[]
end

show(Foo())

causes an error since x will neither be a Base.RefValue nor a String.

This two-line PR should fix this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d1db7a1) 97.50% compared to head (e5ffa70) 97.50%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files           2        2           
  Lines         120      120           
=======================================
  Hits          117      117           
  Misses          3        3           

Files	Coverage Δ
src/ProtoStruct.jl	97.47% <100.00%> (ø)

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[BeastyBlacksmith]

Thanks!