fixed jacobian(f::Fun)

[dstahlke]

Test case (requires also https://github.com/JuliaApproximation/ApproxFunBase.jl/pull/479): using ApproxFun

xdom = Chebyshev(-1..1)
ydom = Chebyshev(-1..1)
domain = xdom * ydom
x,y = Fun(identity, domain)
Dx = Derivative(Chebyshev()^2, [1,0])
Dy = Derivative(Chebyshev()^2, [0,1])

N(u, v) = [
    2*u - x;
    3*v + y
]

u0 = one(x) * one(y)
v0 = one(x) * one(y)
u, v = newton(N, [u0, v0])

Bug report: https://github.com/JuliaApproximation/ApproxFun.jl/issues/887