Caching SVG doesn't cache CSS class name and Viewbox settings

[drpeck]

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There is an issue where caching an SVG with a class name (or Viewbox setting) causes the class name to be contained in the cache. If outputting the same SVG later includes the class name from the first usage.

This PR splits the caching so that the file read is independently cached from the Css class name and Viewbox settings.

[drpeck]

Honestly, I'm pretty confident about this PR but it shouldn't be merged yet. I'm not sure how best to test it. The unit tests all pass but I failed to create new unit tests as I couldn't work out how to mock the AppCache correctly. Any ideas how to do this? Do you also have an existing method of testing this on a 'real' site, or should I work something out for that?