Fix missing html in context

[palmitoto]

Hello,

in the lasts commits, the "html" var was not present in the context when the render is done.

[codecov-io]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #68   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines         175    176    +1     
  Branches        9      9           
=====================================
+ Hits          175    176    +1

Impacted Files	Coverage Δ
djangocms_snippet/cms_plugins.py	100% <100%> (ø)	⬆️
djangocms_snippet/templatetags/snippet_tags.py	100% <0%> (ø)	⬆️
djangocms_snippet/models.py	100% <0%> (ø)	⬆️

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[FinalAngel]

Thanks for the PR @palmitoto