PR for v.1.8.0

[hcpchris]

• BUGFIX: Fixed over-eager trimming of None data values when serializing .to_json() for export via the Export Server. Closes #168.
• BUGFIX: Fixed DumbbellSeries.data incorrect type (reverted from CartesianData to RangeData-descended). Closes #176.
• BUGFIX: Fixed coercion of datetime.date type values to milliseconds when serializaing .to_json() for export via the Export Server.
• ENHANCEMENT: Added for_export as a special parameter to the .to_json() serialization method to maintain past behavior while simultaneously fixing serialization bugs related to export via the Node Export Server. Extended .trim_dict() and .trim_iterable() internal methods with for_export support for the same purpose.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 31.03448% with 20 lines in your changes missing coverage. Please review.

Project coverage is 88.25%. Comparing base (c7a6187) to head (7c271cf). Report is 1 commits behind head on master.

Files	Patch %	Lines
highcharts_core/metaclasses.py	11.11%	14 Missing and 2 partials :warning:
highcharts_core/headless_export.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #178 +/- ## ========================================== - Coverage 88.29% 88.25% -0.05% ========================================== Files 209 209 Lines 30903 30920 +17 Branches 2500 2508 +8 ========================================== + Hits 27285 27287 +2 - Misses 2678 2691 +13 - Partials 940 942 +2 ```

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