PR for v.1.7.6

[hcpchris]

• BUGFIX: Enabled support for the 'image/svg+xml' export type when using the Node Export Server. Now supplying a type of 'svg' will coerce (under the hood) to 'image/svg+xml' which supports more extensive rendering options.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.29%. Comparing base (eff958a) to head (c7a6187). Report is 1 commits behind head on master.

Files	Patch %	Lines
highcharts_core/headless_export.py	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #177 +/- ## ========================================== - Coverage 88.30% 88.29% -0.02% ========================================== Files 209 209 Lines 30901 30903 +2 Branches 2499 2500 +1 ========================================== - Hits 27287 27285 -2 - Misses 2674 2678 +4 Partials 940 940 ```

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