Fix Vararg-UnionAll depwarn

[JackDunnNZ]

Prior to this change, when running with --depwarn=yes, this warning is shown during compilation:

WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).

[codecov]

Codecov Report

Merging #169 (d287f76) into master (cbebbea) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   96.55%   96.55%           
=======================================
  Files           4        4           
  Lines         639      639           
=======================================
  Hits          617      617           
  Misses         22       22           

Impacted Files	Coverage Δ
src/FillArrays.jl	94.81% <100.00%> (ø)

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