Support responseType property on XMLHttpRequest.

Only default, "" and "text" supported for now. Raising an error otherwise.

declarations.d.ts

@@ -37,6 +37,7 @@ declare class XMLHttpRequest {
     overrideMimeType(mime: string): void;
     readyState: number;
     responseText: string;
+    responseType: string;
     status: number;
 
     onload: () => void;