Won't register with fastify

[kristianmandrup]

Please see fastify/fastify#499

[kristianmandrup]

Some problem with TypeScript. Could you make a working test/example with Typescript?? :)

[gj]

Hey @kristianmandrup !

I haven't done much in TS, but if you (or anyone else) wants to contribute some type defs I'd love the PR!