Added Content-Type request header

[pioorg]

Apparently, some web drivers cannot accept requests from the clients if the clients don't set correct content type explicitly.
One example is WinAppDriver. Without this header it simply returns 500 with a vague message:
"status":13, "value":{"error":"unknown error", "message":"Object reference not set to an instance of an object."}}
This single line makes selenium work with WinAppDriver.