[#4265] Not allow to add/set DefaultHttpHeaders to itself.

Motivation:

We should prevent to add/set DefaultHttpHeaders to itself to prevent unexpected side-effects.

Modifications:

Throw IllegalArgumentException if user tries to pass the same instance to set/add.

Result:

No surprising side-effects.

codec-http/src/test/java/io/netty/handler/codec/http/HttpHeadersTest.java

@@ -72,4 +72,16 @@ public class HttpHeadersTest {
         HttpHeaders headers = new DefaultHttpHeaders(false);
         headers.set(of("test"), (CharSequence) null);
     }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testAddSelf() {
+        HttpHeaders headers = new DefaultHttpHeaders(false);
+        headers.add(headers);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testSetSelf() {
+        HttpHeaders headers = new DefaultHttpHeaders(false);
+        headers.set(headers);
+    }
 }

codec/src/main/java/io/netty/handler/codec/DefaultHeaders.java

@@ -540,7 +540,7 @@ public class DefaultHeaders<T> implements Headers<T> {
     public Headers<T> set(Headers<? extends T> headers) {
         checkNotNull(headers, "headers");
         if (headers == this) {
-            return this;
+            throw new IllegalArgumentException("can't add to itself.");
         }
         clear();
         if (headers instanceof DefaultHeaders) {

codec/src/test/java/io/netty/handler/codec/DefaultHeadersTest.java

@@ -379,6 +379,18 @@ public class DefaultHeadersTest {
         assertEquals(headers1, expected);
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testAddSelf() {
+        Headers<ByteString> headers = newInstance();
+        headers.add(headers);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testSetSelf() {
+        Headers<ByteString> headers = newInstance();
+        headers.set(headers);
+    }
+
     private ByteString bs(String value) {
         return new ByteString(value, CharsetUtil.US_ASCII);
     }