Fix maximum(abs, ::Fun)

[jishnub]

Fix JuliaApproximation/ApproxFun.jl#907

After this,

julia> x = Fun()
Fun(Chebyshev(), [0.0, 1.0])

julia> a = abs(x)
Fun(ContinuousSpace{Float64, Float64, PiecewiseSegment{Float64, Vector{Float64}}}(PiecewiseSegment{Float64, Vector{Float64}}([-1.0, -0.0, 1.0])), [1.0, 5.55112e-17, 1.0])

julia> using LinearAlgebra

julia> norm(x - a, Inf)
2.0

julia> norm(a - x, Inf)
2.0

[codecov]

Codecov Report

Patch coverage: 21.73% and project coverage change: -0.03% ⚠️

Comparison is base (c31337a) 27.53% compared to head (74e44f0) 27.51%.
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
- Coverage   27.53%   27.51%   -0.03%     
==========================================
  Files          80       80              
  Lines        8379     8382       +3     
==========================================
- Hits         2307     2306       -1     
- Misses       6072     6076       +4     

Files Changed	Coverage Δ
src/Operators/Operator.jl	51.98% <0.00%> (ø)
src/Operators/banded/CalculusOperator.jl	17.21% <0.00%> (ø)
src/Spaces/SumSpace.jl	31.98% <0.00%> (ø)
src/specialfunctions.jl	16.77% <0.00%> (ø)
src/Operators/general/InterlaceOperator.jl	23.56% <50.00%> (+0.09%)	⬆️

... and 2 files with indirect coverage changes

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