Content-Type defaults to text/plain while the selector defaults to JSON

[Elbarae1921]

When I open the extension, the "type" dropdown defaults to JSON like so:

But when I actually make the request, it's text/plain, so I have to select another type then re-select JSON for the Content-Type header to change.
It's not really much of an issue (I can live with it), but I thought it'd be cool if content-type is JSON by default, or at least have the dropdown point at a blank value or something (so it doesn't mislead you to think it's JSON by default).

[felipevencato]

Same as #172