[#3869] 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

@@ -57,4 +57,16 @@ public class HttpHeadersTest {
         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);
+    }
 }