String formatted timestamps are not correctly detected during import

[ajbarry]

Expected behavior

  describe("unserializeSpecialTypes", () => {
    before(() => {
      admin.initializeApp();
    });

    it("should convert parsable date string timestamps", () => {
      const dateStringTypes = {
        iso8601: {
          __datatype__: "timestamp",
          value: "2020-06-01T04:59:03-07:00"
        }
      };

      const results = unserializeSpecialTypes(dateStringTypes);
      expect(results.iso8601).to.be.an.instanceof(admin.firestore.Timestamp);
    });
  });

Actual behavior

Helpers
       unserializeSpecialTypes
         should convert parsable date string timestamps:
     Error: Value for argument "seconds" is not a valid integer.

Steps to reproduce the behavior

Attempt to import a Date.parse()-able timestamp value.