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

[mjp98]

This addresses #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]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 55.26%. Comparing base (9e3e0bb) to head (60e0736).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/periodicline.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            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     

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