Type-inference improvement in reduce over Ones

[jishnub]

This makes the following concretely type-inferred:

julia> @inferred sum([Ones(4)])
4-element Fill{Float64}, with entries equal to 1.0

Without this, the return type is inferred to be a Union{Ones, Fill}

[codecov]

Codecov Report

Merging #291 (8ffaf5e) into master (a718e2f) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files           5        5           
  Lines         869      872    +3     
=======================================
+ Hits          868      871    +3     
  Misses          1        1           

Files Changed	Coverage Δ
src/FillArrays.jl	99.74% <100.00%> (+<0.01%)	⬆️
src/fillalgebra.jl	100.00% <100.00%> (ø)

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