Fixed bugs in .to_dict() and .to_json().

[hcpchris]

Closes #74 and #75.

[codecov]

Codecov Report

Merging #79 (42b3ed6) into develop (1648af1) will decrease coverage by 0.01%.
Report is 13 commits behind head on develop.
The diff coverage is 74.46%.

@@             Coverage Diff             @@
##           develop      #79      +/-   ##
===========================================
- Coverage    90.28%   90.28%   -0.01%     
===========================================
  Files          206      206              
  Lines        27516    27593      +77     
  Branches      1794     1820      +26     
===========================================
+ Hits         24843    24912      +69     
- Misses        1982     1985       +3     
- Partials       691      696       +5     

Files Changed	Coverage Δ
highcharts_core/options/series/data/bar.py	80.08% <0.00%> (-0.35%)	⬇️
highcharts_core/options/series/data/boxplot.py	78.16% <0.00%> (-0.56%)	⬇️
highcharts_core/options/series/data/bullet.py	63.76% <0.00%> (-0.94%)	⬇️
highcharts_core/options/series/data/range.py	79.51% <0.00%> (-0.49%)	⬇️
highcharts_core/options/series/data/vector.py	66.66% <0.00%> (-3.51%)	⬇️
highcharts_core/options/series/data/cartesian.py	71.61% <33.33%> (-0.71%)	⬇️
highcharts_core/metaclasses.py	66.75% <84.00%> (+3.16%)	⬆️
highcharts_core/utility_functions.py	40.09% <85.71%> (+3.14%)	⬆️
highcharts_core/chart.py	43.18% <91.30%> (+3.06%)	⬆️
highcharts_core/constants.py	95.76% <100.00%> (+0.03%)	⬆️
... and 3 more