Add unit test to ensure adding null header values is not allowed.

Motivation:

We need to ensure we never allow to have null values set on headers, otherwise we will see a NPE during encoding them.

Modifications:

Add unit test that shows we correctly handle null values.

Result:

Verify correct implementation.

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

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2013 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.handler.codec.http;
+
+import io.netty.util.AsciiString;
+
+import org.junit.Test;
+
+import java.util.List;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+
+public class HttpHeadersTest {
+
+    @Test
+    public void testRemoveTransferEncodingIgnoreCase() {
+        HttpMessage message = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
+        message.headers().set(HttpHeaderNames.TRANSFER_ENCODING, "Chunked");
+        assertFalse(message.headers().isEmpty());
+        HttpHeaderUtil.setTransferEncodingChunked(message, false);
+        assertTrue(message.headers().isEmpty());
+    }
+
+    // Test for https://github.com/netty/netty/issues/1690
+    @Test
+    public void testGetOperations() {
+        HttpHeaders headers = new DefaultHttpHeaders();
+        headers.add("Foo", "1");
+        headers.add("Foo", "2");
+
+        assertEquals("1", headers.get("Foo"));
+
+        List<CharSequence> values = headers.getAll("Foo");
+        assertEquals(2, values.size());
+        assertEquals("1", values.get(0));
+        assertEquals("2", values.get(1));
+    }
+
+    @Test
+    public void testEquansIgnoreCase() {
+        assertThat(AsciiString.equalsIgnoreCase(null, null), is(true));
+        assertThat(AsciiString.equalsIgnoreCase(null, "foo"), is(false));
+        assertThat(AsciiString.equalsIgnoreCase("bar", null), is(false));
+        assertThat(AsciiString.equalsIgnoreCase("FoO", "fOo"), is(true));
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void testSetNullHeaderValueValidate() {
+        HttpHeaders headers = new DefaultHttpHeaders(true);
+        headers.set("test", (CharSequence) null);
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void testSetNullHeaderValueNotValidate() {
+        HttpHeaders headers = new DefaultHttpHeaders(false);
+        headers.set("test", (CharSequence) null);
+    }
+}