tm-http-request: Add support for binary responses

With a demo courtesy of https://random.dog/

@rmunn you recently worked on the base64 utilities. I tried to use $tw.utils.base64Encode instead of window.btoa, but found that it didn't work. It's concerning because we expose that utility method as a filter operation, and it would be frustrating if we were not base64encoding things properly.

core/modules/startup/rootwidget.js

@@ -38,6 +38,7 @@ exports.startup = function() {
 			url: params.url,
 			method: params.method,
 			body: params.body,
+			binary: params.binary,
 			oncompletion: params.oncompletion,
 			onprogress: params.onprogress,
 			bindStatus: params["bind-status"],