Refactor handling of null request values as per PR comments. See: https://github.com/dart-lang/http/pull/77/files/26e6075d93a41f381012b1588b2f4ccb056e9231

Author: bobjackman

lib/src/utils.dart

@@ -13,9 +13,8 @@ import 'byte_stream.dart';
 ///     mapToQuery({"foo": "bar", "baz": "bang"});
 ///     //=> "foo=bar&baz=bang"
 String mapToQuery(Map<String, String> map, {Encoding encoding}) {
-  map.keys
-    .where((k) => (map[k] == null)).toList() // -- keys for null elements
-    .forEach(map.remove);
+  var nullKeys = map.keys.where((k) => (map[k] == null)).toList();
+  nullKeys.forEach(map.remove); // -- remove elements with null values
 
   var pairs = <List<String>>[];
   map.forEach((key, value) =>