Improve accuracy of delta_h for small differences

[kimikage]

This avoids the loss of significance when the difference in hue is small. (cf. #477 (comment))
This results in a slowdown of delta_h, but it is negligible compared to the total cost of DE_2000.

This is one of the preparations for PR #494.

julia> a = Lab(40, 4, 80); b = Lab(50, 5, 101);

julia> Colors.delta_h(a, b)
-0.043853227f0 # master
-0.04444444f0  # this PR

julia> Colors.delta_h(Lab{BigFloat}(a), Lab{BigFloat}(b))
-0.04444444308963756431578907136117476345617361364807484182394389220997687609203497

[codecov]

Codecov Report

Merging #501 (0e7099b) into master (219f387) will decrease coverage by 0.03%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   93.64%   93.60%   -0.04%     
==========================================
  Files           9        9              
  Lines        1101     1111      +10     
==========================================
+ Hits         1031     1040       +9     
- Misses         70       71       +1     

Impacted Files	Coverage Δ
src/utilities.jl	98.64% <92.85%> (-0.41%)	⬇️

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