BUGFIX: #124 ChartOptions.margin and .spacing deserialization

[hcpchris]

Closes #124 .

Turns out the issue was an edge case that occurred when:

1. ChartOptions were first serialized to a dict using .to_dict().
2. ChartOptions were then deserialized using .from_dict().

The error had to do with how 'null' values in the serialized margin/spacing property were handled.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (59ad403) 88.26% compared to head (72c1897) 88.26%.
Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #125      +/-   ##
===========================================
- Coverage    88.26%   88.26%   -0.01%     
===========================================
  Files          208      208              
  Lines        30217    30239      +22     
  Branches      2409     2420      +11     
===========================================
+ Hits         26671    26690      +19     
- Misses        2646     2647       +1     
- Partials       900      902       +2     

Files	Coverage Δ
highcharts_core/options/chart/__init__.py	92.20% <88.88%> (-0.23%)	⬇️

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