code: function(field, val){
              if (2 == arguments.length) {
                  if (Array.isArray(val)) val = val.map(String);
                  else val = String(val);
                  this.res.setHeader(field, val);
              } else {
                  for (var key in field) {
                      this.set(key, field[key]);
                  }
              }
          }
docs: Set header `field` to `val`, or pass
  an object of header fields.
  
  Examples:
  
  this.set('Foo', ['bar', 'baz']);
  this.set('Accept', 'application/json');
  this.set({ Accept: 'text/plain', 'X-API-Key': 'tobi' });
  
  @param {String|Object|Array} field
  @param {String} val
  @api public