Fix analyzer warnings in http_multi_server.

pkg/http_multi_server/lib/src/multi_headers.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library http_multi_server.multi_headers;
 
 import 'dart:io';
 
 /// A class that delegates header access and setting to many [HttpHeaders]
 /// instances.
@@ skipping 77 matching lines @@
       _headers.first.forEach(f);
 
   void noFolding(String name) {
     for (var headers in _headers) {
       headers.noFolding(name);
     }
   }
 
   void remove(String name, Object value) {
     for (var headers in _headers) {
-      headers.remove(name);
+      headers.remove(name, value);
     }
   }
 
   void removeAll(String name) {
     for (var headers in _headers) {
-      headers.removeAll(name, value);
+      headers.removeAll(name);
     }
   }
 
   void set(String name, Object value) {
     for (var headers in _headers) {
       headers.set(name, value);
     }
   }
 
   String value(String name) => _headers.first.value(name);
 
   List<String> operator[](String name) => _headers.first[name];
+
+  void clear() {
+    for (var headers in _headers) {
+      headers.clear();
+    }
+  }
 }