Fix differentiate of AbstractArrays

Previously `differentiate([x+y,y+z],[x,y,z])` would return a 3 x 2 
matrix instead of the more commonly used 2 x 3 matrix.

Author: saschatimme

src/differentiation.jl

@@ -40,11 +40,11 @@ differentiate(p::RationalPoly, v::AbstractVariable) = (differentiate(p.num, v) *
 const ARPL = Union{APL, RationalPoly}
 
 function differentiate(ps::AbstractArray{PT}, xs::AbstractArray) where {PT <: ARPL}
-    differentiate.(reshape(ps, (1, size(ps)...)), reshape(xs, :))
+    differentiate.(reshape(ps, (size(ps)..., 1)), reshape(xs, 1, :))
 end
 
 function differentiate(ps::AbstractArray{PT}, xs::Tuple) where {PT <: ARPL}
-    differentiate.(reshape(ps, (1, size(ps)...)), xs)
+    differentiate(ps, collect(xs))
 end
 
 

test/differentiation.jl

@@ -1,5 +1,5 @@
 @testset "Differentiation" begin
-    Mod.@polyvar x y
+    Mod.@polyvar x y z
     @test differentiate(3, y) == 0
     @test differentiate.([x, y], y) == [0, 1]
     @test differentiate([x, y], (x, y)) == Matrix(1.0I, 2, 2) # TODO: this can be just `I` on v0.7 and above
@@ -34,6 +34,10 @@
     @test isa(p, Matrix{<:AbstractPolynomial{Float64}})
     @test p == [4.0 6.0y; 6.0y 6.0x+24.0y]
 
+    f = [x^2+y, z^2+4x]
+    @test differentiate(f, [x, y, z]) == [2x 1 0; 4 0 2z]
+    @test differentiate(f, (x, y, z)) == [2x 1 0; 4 0 2z]
+
     @testset "differentiation with Val{}" begin
         @test @inferred(differentiate(x, x, Val{0}())) == x
         @test @inferred(differentiate(x, x, Val{1}())) == 1