Protocol: add "Content-type" header to "queries" POST request

[brcarp]

This isn't quite a bug or a feature request, but if the POST requests to the queries endpoint could include a Content-Type: application/json header, it might help browsers and debuggers parse the payload from the user to aid in debugging applications using instantsearch. Some tools trip up on this and expect, absent any cues from Content-type, that the payload is application/x-www-form-urlencoded (because some of the properties like params are in fact URL-encoded) and presents it improperly parsed to the investigator.

[Haroenv]

This is done on purpose, to ensure the request is a "Simple Request" which doesn't need the CORS preflight request, you can learn more about that here: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests

It's also annoying for us to debug (we're thinking about transitioning to a plain text request in v5), but in the mean time I also made a tool to inspect the parameters: https://haroen.me/algolia-tools/#request-explorer