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]

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 ⚠️

Additional details and impacted files

@@            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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