Oja hg fix h2

This contains a fix for a bug when Chrome inserts extra text elements after a change of focus (i.e. a button click) and leaves unwanted H2 elements in the remaining markup

See the large comment above the change code for more.

We should leave a comment to note what bug this test is *trying* to re-produce.

We're currently linking tests with patches just by duplicating comments and the links to isolated test cases. This could be done better: https://github.com/guardian/scribe/issues/93#issuecomment-46289370