added correct Content-Type and small syntax fixs

[elmit]

[Thibauth]

Thanks a lot. I agree that using Content-Type: application/json would be semantically better. However, if I remember correctly, a request with a Content-Type of application/json is no longer considered a "simple" request by the browser and is blocked by the same-origin policy.

Can you confirm/check that the requests sent by the extension still go through even when the Content-Type is set to application/json. Or do you assume that the server the requests are sent to will always support CORS?