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/utils.js
if (obj === null || typeof obj === 'undefined') {
    return;
  }
  
  if (typeof obj !== 'object') {
    
    obj = [obj];
  }
  if (isArray(obj)) {