Fixing HTML serialization of embed fragments

Serialization of embed fragments was not working in the JS kit. It's impossible to write a real-life test for it, since we don't have an example using embed fragments.

Note: serialization of embed blocks (inside structured text fragments) works, has always worked, and has always been properly tested using [this blog post on LesBonnesChoses](http://lesbonneschoses.prismic.me/blog/UkL0gMuvzYUANCpr/get-the-right-approach-to-ganache).