Fix element type of gradient initialization in HessianResult

Author: andreasnoack

src/DiffResults.jl

@@ -101,7 +101,7 @@ Note that `HessianResult` allocates its own storage; `x` is only used for type a
 shape information. If you want to allocate storage yourself, use the `DiffResult`
 constructor instead.
 """
-HessianResult(x::AbstractArray) = DiffResult(first(x), zeros(size(x)), similar(x, length(x), length(x)))
+HessianResult(x::AbstractArray) = DiffResult(first(x), zeros(eltype(x), size(x)), similar(x, length(x), length(x)))
 HessianResult(x::StaticArray) = DiffResult(first(x), x, zeros(StaticArrays.similar_type(typeof(x), Size(length(x),length(x)))))
 
 #############

test/runtests.jl

@@ -293,5 +293,7 @@ using DiffResults: DiffResult, GradientResult, JacobianResult, HessianResult,
         @test size(gradient(HessianResult(x0))) == size(x0)
         @test size(gradient(HessianResult(x1))) == size(x1)
         @test size(gradient(HessianResult(x2))) == size(x2)
+
+        @test HessianResult(Float32.(x0)).derivs[1] isa Vector{Float32}
     end
 end