UnsetSpace compatibility with other spaces

[jishnub]

Fixes JuliaApproximation/ApproxFun.jl#616

julia> I = Interval(0,1); x = Fun(identity, I); ∂ = Derivative(I);

julia> ∂ * Multiplication(x)
TimesOperator : ApproxFunBase.UnsetSpace() → Ultraspherical(1,0..1)

julia> ∂ * Multiplication(x) : Chebyshev(I)
TimesOperator : Chebyshev(0..1) → Ultraspherical(1,0..1)
 1.0  1.0  0.5   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅   1.0  2.0  1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅   1.5  3.0  1.5   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅   2.0  4.0  2.0   ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅   2.5  5.0  2.5   ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅   3.0  6.0  3.0   ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   3.5  7.0  3.5   ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   4.0  8.0  4.0  ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   4.5  9.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   5.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱

julia> ∂ * Multiplication(x) * Fun(Legendre(I)) ≈ 2x
true

[codecov]

Codecov Report

Merging #145 (6cf64e7) into master (6cf64e7) will not change coverage.
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  Lines        7741     7741           
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  Hits         1192     1192           
  Misses       6549     6549           

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