Preserve eltype in converting Symmetric to Matrix

[jishnub]

This allows cases where the eltype differs from that of the parent:

julia> M = [UpperTriangular([1 2; 3 4]) for i in 1:2, j in 1:2]
2×2 Matrix{UpperTriangular{Int64, Matrix{Int64}}}:
 [1 2; 0 4]  [1 2; 0 4]
 [1 2; 0 4]  [1 2; 0 4]

julia> H = Hermitian(M)
2×2 Hermitian{AbstractMatrix, Matrix{UpperTriangular{Int64, Matrix{Int64}}}}:
 [1 2; 2 4]  [1 2; 0 4]
 [1 0; 2 4]  [1 2; 2 4]

julia> Array(H)
2×2 Matrix{AbstractMatrix}:
 [1 2; 2 4]  [1 2; 0 4]
 [1 0; 2 4]  [1 2; 2 4]

This conversion throws an error at present on master.