error in convert for Diagonal

[JeffBezanson]

Playing with #563, I discovered this:

julia> convert(Diagonal{Int,Vector{Int}}, Diagonal(1:4))
ERROR: MethodError: no method matching Array{Int64,1}(::Diagonal{Int64,UnitRange{Int64}})

The reason is that the only constructor for instantiated Diagonal types like Diagonal{Int,Vector{Int}} (the "inner" constructor) expects its argument to be a vector. Diagonal(x) accepts both vectors and matrices.

This could be fixed by adding another inner constructor. Or we could add convert methods, since it seems at least conversion should work. Also we could add ::AbstractVector to the argument of the current inner constructor.

[mcognetta]

These should be defined individually for all of these matrices (Bidiagonal, Tridiagonal, etc) right?
If this is the case, adding

Diagonal{T, V}(A::Diagonal) where {T, V<:AbstractVector{T}} = Diagonal(convert(V, A.diag)) and analogous ones for the other types should be sufficient.

[ViralBShah]

julia> convert(Diagonal{Int,Vector{Int}}, Diagonal(1:4))
4×4 Diagonal{Int64, Vector{Int64}}:
 1  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4