Fixing CorsConfigTest failure under Java 8.

Motivation:
When running the build with Java 8 the following error occurred:

java: reference to preflightResponseHeader is ambiguous
  both method
  <T>preflightResponseHeader(java.lang.CharSequence,java.lang.Iterable<T>)
  in io.netty.handler.codec.http.cors.CorsConfig.Builder and method
  <T>preflightResponseHeader(java.lang.String,java.util.concurrent.Callable<T>)
  in io.netty.handler.codec.http.cors.CorsConfig.Builder match

The offending class was CorsConfigTest and its shouldThrowIfValueIsNull
which contained the following line:
withOrigin("*").preflightResponseHeader("HeaderName", null).build();

Modifications:
Updated the offending method with to supply a type, and object array, to
avoid the error.

Result:
After this I was able to build with Java 7 and Java 8

codec-http/src/test/java/io/netty/handler/codec/http/cors/CorsConfigTest.java

@@ -101,7 +101,7 @@ public class CorsConfigTest {
 
     @Test (expected = IllegalArgumentException.class)
     public void shouldThrowIfValueIsNull() {
-        withOrigin("*").preflightResponseHeader("HeaderName", null).build();
+        withOrigin("*").preflightResponseHeader("HeaderName", new Object[]{null}).build();
     }
 
 }