fix arrow saver

Author: timkpaine

packages/perspective-jupyterlab/src/ts/renderer.ts

@@ -104,9 +104,10 @@ export class PerspectiveDocumentWidget extends DocumentWidget<PerspectiveWidget>
                         this.context.save();
                     });
                 } else if (this._type === "arrow") {
-                    view.to_arrow().then((result: any) => {
+                    view.to_arrow().then((result: ArrayBuffer) => {
                         console.log(result);
-                        this.context.model.fromString(result);
+                        const resultAsB64 = btoa(new Uint8Array(result).reduce((acc, i) => (acc += String.fromCharCode.apply(null, [i])), ""));
+                        this.context.model.fromString(resultAsB64);
                         this.context.save();
                     });
                 } else if (this._type === "json") {