Response.getHeaders(String name) should return an empty list instead of null, close AsyncHttpClient#764

Author: Stephane Landelle

api/src/main/java/org/asynchttpclient/providers/ResponseBase.java

@@ -67,7 +67,7 @@ public final String getHeader(String name) {
 
     @Override
     public final List<String> getHeaders(String name) {
-        return headers != null ? getHeaders().get(name) : null;
+        return headers != null ? getHeaders().get(name) : Collections.<String> emptyList();
     }
 
     @Override