A key for request attribute that contains any exception that might have occured
before the handling has been propagated to SkinnyMicroBase#handle (such as in FileUploadSupport)
valForceHttpsKey: String
valHostNameKey: String
valIsAsyncKey: String
valPortKey: String
valRenderCallbacks: String
defaddCallback(callback: (Try[Any]) ⇒ Unit)(implicit ctx: SkinnyContext): Unit
defaddRenderCallback(callback: (Try[Any]) ⇒ Unit)(implicit ctx: SkinnyContext): Unit