Remove specialized issymmetric/ishermitian for Diagonal{<:Number}

[jishnub]

Unfortunately, this definition doesn't work if the matrix contains NaN, as it is not equal to itself. It is better to fall back to checking each element by default.

This fixes the mismatch in

julia> D = Diagonal(fill(NaN,2))
2×2 Diagonal{Float64, Vector{Float64}}:
 NaN       ⋅ 
    ⋅   NaN

julia> issymmetric(D)
true

julia> issymmetric(Array(D))
false

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.92%. Comparing base (b464203) to head (aaf1f0f).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
+ Coverage   91.88%   91.92%   +0.03%     
==========================================
  Files          34       34              
  Lines       15374    15372       -2     
==========================================
+ Hits        14127    14131       +4     
+ Misses       1247     1241       -6     

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