[ENHANCEMENT] Map/broadcast doesn't preserve certain diagonal containers

[mtfishman]

For example:

julia> using DiagonalArrays: DiagonalArrays, DiagonalArray, diagview

julia> using FillArrays: Fill

julia> DiagonalArray(Fill(2, 2), (2, 2))
2×2 DiagonalArray{Int64, 2, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}, typeof(SparseArraysBase.default_getunstoredindex)}:
 2    ⋅
  ⋅  2

julia> diagview(a)
2-element Fill{Int64}, with entries equal to 2

julia> 2a
2×2 DiagonalArray{Int64, 2, Vector{Int64}, typeof(SparseArraysBase.default_getunstoredindex)}:
 4    ⋅
  ⋅  4

julia> diagview(2a)
2-element Vector{Int64}:
 4
 4

julia> pkgversion(DiagonalArrays)
v"0.2.2"

[mtfishman]

Related to #6.