onRequest method
Called before the request is sent. Mutate handler.context or call
handler.reject to abort.
Implementation
@override
void onRequest(RequestContext context, RequestHandler handler) {
_log('→ ${context.method} ${context.uri}');
if (context.headers.isNotEmpty) _log(' headers: ${context.headers}');
if (context.body != null) _log(' body: ${context.body}');
handler.next(context);
}