Fix coefficients for ProductFun

[jishnub]

After this, the following works:

julia> P = ProductFun((x,y) -> x*y, Chebyshev()^2)
ProductFun on Chebyshev() ⊗ Chebyshev()

julia> coefficients(P, space(P))
2×2 Matrix{Float64}:
 0.0  0.0
 0.0  1.0

[codecov[bot]]

Codecov Report

Patch coverage: 10.52% and project coverage change: -0.01% :warning:

Comparison is base (27c3236) 27.41% compared to head (5658d57) 27.41%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #535 +/- ## ========================================== - Coverage 27.41% 27.41% -0.01% ========================================== Files 80 80 Lines 8363 8368 +5 ========================================== + Hits 2293 2294 +1 - Misses 6070 6074 +4 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Multivariate/ProductFun.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL011bHRpdmFyaWF0ZS9Qcm9kdWN0RnVuLmps) | `0.00% <0.00%> (ø)` | | | [src/eigen.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2VpZ2VuLmps) | `0.00% <0.00%> (ø)` | | | [src/Spaces/QuotientSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9RdW90aWVudFNwYWNlLmps) | `3.95% <28.57%> (+0.50%)` | :arrow_up: |

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