Fix updated lints (#91)

diff --git a/lib/src/http_headers_impl.dart b/lib/src/http_headers_impl.dart
index 6e04eda..9f2dff7 100644
--- a/lib/src/http_headers_impl.dart
+++ b/lib/src/http_headers_impl.dart
@@ -1128,7 +1128,9 @@
   int year = toInt(yearStr);
   if (year >= 70 && year <= 99) {
     year += 1900;
-  } else if (year >= 0 && year <= 69) year += 2000;
+  } else if (year >= 0 && year <= 69) {
+    year += 2000;
+  }
   if (year < 1601) error();
 
   int dayOfMonth = toInt(dayOfMonthStr);