isObject should return false for Date as well as File, null, and Array

Without this, assigning dates to the form's value instead merges them with a default value for objects of `{}` (in mergeDefaultsWithFormData), which stops them being dates.

packages/utils/src/isObject.ts

@@ -8,5 +8,8 @@ export default function isObject(thing: any) {
   if (typeof File !== "undefined" && thing instanceof File) {
     return false;
   }
+  if (typeof Date !== "undefined" && thing instanceof Date) {
+    return false;
+  }
   return typeof thing === "object" && thing !== null && !Array.isArray(thing);
 }