ProductSpace domain type fix

[jishnub]

After this,

julia> f = Fun();

julia> P = ProductFun([f, f, f], space(f));

julia> typeof(domain(P)) == ApproxFunBase.domaintype(space(P))
true

[codecov]

Codecov Report

Base: 71.90% // Head: 71.97% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (f562a84) compared to base (5da41bb).
Patch coverage: 86.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
+ Coverage   71.90%   71.97%   +0.06%     
==========================================
  Files          79       79              
  Lines        8301     8311      +10     
==========================================
+ Hits         5969     5982      +13     
+ Misses       2332     2329       -3     

Impacted Files	Coverage Δ
src/Spaces/SumSpace.jl	71.31% <33.33%> (ø)
src/LinearAlgebra/helper.jl	64.23% <100.00%> (+0.08%)	⬆️
src/Multivariate/TensorSpace.jl	87.50% <100.00%> (+1.17%)	⬆️

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