reset haxe.HttpBase.responseAsString on request

fixes #8894

std/flash/_std/haxe/Http.hx

@@ -41,6 +41,7 @@ class HttpFlash extends haxe.http.HttpBase {
 	}
 
 	public override function request(?post:Bool) {
+		responseAsString = null;
 		responseBytes = null;
 		var loader = req = new flash.net.URLLoader();
 		loader.dataFormat = BINARY;

std/haxe/http/HttpBase.hx

@@ -231,6 +231,7 @@ class HttpBase {
 
 	function success(data:Bytes) {
 		responseBytes = data;
+		responseAsString = null;
 		if (hasOnData()) {
 			onData(responseData);
 		}