code: function Request_addHeader(key, value) {
          var item;
          if (Type.isString(key)) {
              key = key.toLowerCase();
              if (!Type.isString(value) && !!value && Type.isFunction(value.toString)) {
                  value = value.toString();
              }
              if (Type.isString(value)) {
                  if (this.hasHeader(key) && !Type.isArray(this.headers[key])) {
                      item = this.getHeader(key);
                      this.headers[key] = [];
                      this.headers[key].push(item);
                      this.headers[key].push(value);
                  } else if (this.hasHeader(key) && Type.isArray(this.headers[key])) {
                      this.headers[key].push(value);
                  } else {
                      this.headers[key] = value;
                  }
              }
          } else {
              throw new error.HttpError(500, {
                  key: key,
                  value: value
              }, "Request.addHeader: Header key must be string type");
          }
      }
docs: @since 0.0.1
  @author Igor Ivanovic
  @method Request#addHeader
  
  @description
  Write header