<img> element should inherit border-radius from its parent. Fix #90

[derfel]

I don't know if you want to add a specific mixin for the img element, but I think in the meantime you could apply this little patch to let fix #90.

If the img element inherit border-radius from its parent there's no need for a new mixin and it works with both shady and shadow dom.

For example:
iron-icon { --iron-icon: { border-radius: 50%; }; }

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[derfel]

I signed it!

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CLAs look good, thanks!

[dominik0711]

Is there a road-map when the patch from @derfel will be merged into the upcoming release?