code: function (header) {
          if (!(header && header.key)) { return; } // if no valid header is provided, do nothing
  
          var existing = this.headers.find({ key: header.key });
  
          if (!existing) {
              return this.headers.add(header);
          }
  
          existing.value = header.value;
      }
docs: Updates or inserts the given header.
  
  @param {Object} header