Merging CSS rules with same properties fails in spec test

In the following spec test, you can see that there's two CSS rules that should have been merged together but weren't:

https://github.com/ampproject/amp-toolbox/blob/1cf9646b6723b4a3556356da02be190caacd1f32/packages/optimizer/spec/transformers/valid/ServerSideRendering/converts_sizes_attribute_to_css/expected_output.html#L8

A single rule `#i-amp-0,#i-amp-1{width:100vw}` should have been generated instead of the two separate rules `#i-amp-0{width:100vw}` & `#i-amp-1{width:100vw}`.

This currently causes a failure in the PHP library tests here: https://travis-ci.org/github/ampproject/amp-wp/jobs/693721158#L950-L951