configure method
Allows more advanced configuration of this request prior to sending.
The supplied callback configureRequest
should be called after opening,
but prior to sending, this request. The request
parameter will either
be an instance of HttpRequest or HttpClientRequest,
depending on the w_transport usage. If configureRequest
returns a Future,
the request will not be sent until the returned Future completes.
Implementation
@override
void configure(configure(Object request)) {
verifyUnsent();
configureFn = configure;
}