code: function (headers, headerKey, defaultValue) {
          var headerName = _.findKey(headers, function (value, key) {
              return key.toLowerCase() === headerKey.toLowerCase();
          });
  
          if (!headerName) {
              headers[headerKey] = defaultValue;
          }
      }
docs: Checks if a header already exists. If it does not, sets the value to whatever is passed as
  `defaultValue`
  
  @param {object} headers
  @param {String} headerKey
  @param {String} defaultValue