Call `domain(sp)` rather than `domain(cfs)` in PeriodicLine hilbert

[mjp98]

This addresses https://github.com/JuliaApproximation/SingularIntegralEquations.jl/issues/152 by correcting calls of domain(f::AbstractVector) to domain(S::Space) in hilbert(S::Space{<:PeriodicLine},f::AbstractVector,z::Number)

Is it worth adding tests? e.g.

using ApproxFun, SingularIntegralEquations, Test
F = Fun(z->1/(1+z^2),PeriodicLine())
hilbertF = z-> -z/(1+z^2)
z = pi
@test hilbert(F,z) ≈ hilbertF(z)

[codecov[bot]]

Codecov Report

Merging #153 (60e0736) into master (9e3e0bb) will decrease coverage by 0.02%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   55.29%   55.26%   -0.03%     
==========================================
  Files          34       34              
  Lines        2400     2401       +1     
==========================================
  Hits         1327     1327              
- Misses       1073     1074       +1     

Impacted Files	Coverage Δ
src/periodicline.jl	15.00% <0.00%> (ø)
src/GreensFun/CauchyWeight.jl	23.80% <0.00%> (-0.59%)	:arrow_down:

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