Fix mul_coefficients for wrapper types

[jishnub]

After this,

julia> mul_coefficients(view(Conversion(Chebyshev(), Legendre()), 1:3, 1:2), Float64[1,2])
3-element Vector{Float64}:
 1.0
 1.9999999999999998
 0.0

[codecov]

Codecov Report

Patch coverage: 89.83% and project coverage change: -0.73 ⚠️

Comparison is base (2f401fa) 71.22% compared to head (ca44d15) 70.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   71.22%   70.50%   -0.73%     
==========================================
  Files          80       81       +1     
  Lines        8331     8470     +139     
==========================================
+ Hits         5934     5972      +38     
- Misses       2397     2498     +101     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	84.00% <ø> (+0.66%)	⬆️
src/LinearAlgebra/blas.jl	77.77% <ø> (ø)
src/Spaces/Spaces.jl	63.52% <ø> (-3.53%)	⬇️
src/Operators/qr.jl	57.89% <22.22%> (-2.82%)	⬇️
src/Spaces/SumSpace.jl	70.44% <33.33%> (+0.77%)	⬆️
src/Operators/SubOperator.jl	82.35% <50.00%> (-0.82%)	⬇️
src/Operators/banded/CalculusOperator.jl	75.40% <80.64%> (+0.84%)	⬆️
src/Operators/functionals/CalculusFunctional.jl	79.48% <86.66%> (+7.07%)	⬆️
src/eigen.jl	96.42% <96.42%> (ø)
src/show.jl	95.68% <97.56%> (+12.85%)	⬆️
... and 6 more

... and 44 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.