Handle intra-selector whitespace ("h1 ,h2"=="h1,h2")

Summary: These are equivalent. We didn't used to treat it that way

Test Plan:
- Add Test
- run test suite

Reviewers: john, pamela

Reviewed By: pamela

Subscribers: #cs

Maniphest Tasks: T3520

Differential Revision: https://phabricator.khanacademy.org/D15268