Restrict Diagonal sqrt branch to positive diag

[jishnub]

As noted in #1193, the sqrt(::Digonal{<:Real}) method requires the diagonal element to be positive, so that sqrt is defined for the individual elements. We therefore may restrict the diagonal branch in the dense sqrt method to matrices with positive diag if the eltype is Real.

Fixes #1193

After this,

julia> A = diagm(0 => [1.0, -1.0])
2×2 Matrix{Float64}:
 1.0   0.0
 0.0  -1.0

julia> sqrt(A)
2×2 Matrix{ComplexF64}:
 1.0+0.0im  0.0+0.0im
 0.0+0.0im  0.0+1.0im

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.86%. Comparing base (2a1696a) to head (751b7ea).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1203   +/-   ##
=======================================
  Coverage   91.86%   91.86%           
=======================================
  Files          34       34           
  Lines       15365    15365           
=======================================
  Hits        14115    14115           
  Misses       1250     1250           

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