Add missing convert methods from Fill to Zeros/Ones

[jishnub]

After this,

julia> convert(Zeros{Float64}, Fill(0,2))
2-element Zeros{Float64}

julia> convert(Zeros, Fill(0,2))
2-element Zeros{Int64}

julia> convert(Ones, Fill(0,2))
ERROR: ArgumentError: cannot convert an array containinig 0 to Ones

This PR addresses #282 (comment)

[codecov]

Codecov Report

Merging #284 (30675b5) into master (41715aa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #284   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files           5        5           
  Lines         851      863   +12     
=======================================
+ Hits          850      862   +12     
  Misses          1        1           

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

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