Content-Type header invalid, use Content-Type: application\/json

Hi,

I'm writing a POST code in Google Sheets script that will go through a test run export populated with results and push it into TR.
My code is as follows:
[..]
function constructApi() {
  var headers =
      {
        'Content-Type': 'application\/json',
        'Authorization': 'Basic ' + Utilities.base64Encode('<username>' + ':' + '<password>'),
      };
  var data =
      {
        "status_id":"1",
        "comments":"Just testing stuff",
        "assignedto_id":"1",
      };
  var payload = JSON.stringify(data);
 
  //also method and specifying the headers above
  var options =
      {
        "method":"POST",
        "headers" : headers,
        "payload" : payload,
      }; 
 
  var runId = resolveRunId();
  //Logger.log(options.results[0].case_id);
  var response = UrlFetchApp.fetch("https://<>.testrail.net/index.php?/api/v2/add_result_for_case/94/341", options);

[..]

but I'm getting the following -
[..]
Request failed for https://<>.testrail.net/index.php?/api/v2/add_result_for_case/94/341 returned code 400. Truncated server response: {"error":"Content-Type header invalid (use Content-Type: application\/json)"} (use muteHttpExceptions option to examine full response) (line 46, file "submitToTestRail")
[..]

Content-Type is in fact application/json so it's not clear what's the problem. The same method, only with GET, is used in another script and it's running just fine.

Thanks.