If a header has been set as a default, there does not appear to be any way to skip it on an individual request. Setting `null` or `undefined` doesn't do anything.

lib/core/dispatchRequest.js
function cleanHeaderConfig(method) {
      delete config.headers[method];
    }
  );
  var adapter = config.adapter || defaults.adapter;
  return adapter(config).then(function onAdapterResolution(response) {
    throwIfCancellationRequested(config);
    
    response.data = transformData(
      response.data,
      response.headers,
      config.transformResponse
    );
    return response;
  }, function onAdapterRejection(reason) {
    if (!isCancel(reason)) {
      throwIfCancellationRequested(config);
      
      if (reason && reason.response) {
        reason.response.data = transformData(
          reason.response.data,
          reason.response.headers,
          config.transformResponse
        );
      }
    }
    return Promise.reject(reason);
  });
};