Relax Fill return type in mvlognormal tests

[jishnub]

In JuliaArrays/FillArrays.jl#385 I'm working on improving how broadcasting works for FillArrays. One of the changes is that some types may be inferred differently, so e.g.
Currently

julia> exp.(Zeros(2))
2-element Fill{Float64}, with entries equal to 1.0

Proposed:

julia> exp.(Zeros(2))
2-element Ones{Float64}

Currently, in one of the mvlognormal tests, this package is explicitly checking for a Fill. This PR relaxes this to a Union{Fill, Ones} so that the tests pass either way.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.99%. Comparing base (00b7fad) to head (fdae203).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1890   +/-   ##
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  Coverage   85.99%   85.99%           
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  Files         144      144           
  Lines        8666     8666           
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  Hits         7452     7452           
  Misses       1214     1214           

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