Send request type as `application/json`

[alexfernandez]

Great little project! The browser part in particular is quite useful. I have a couple of improvements in mind.

I see that it's sending the content type as text/plain. As it's actually sending JSON, I think that application/json would be more appropriate, and would make my server of choice (Fastify) parse it automatically.

Is the project still alive? It would be great to know before taking the time to contribute a pull request or open more issues. Thanks in advance!