FluentCaseInsensitiveStringsMap.get should return an empty immutable list instead of null, close AsyncHttpClient#758

Author: Stephane Landelle

src/main/java/com/ning/http/client/FluentCaseInsensitiveStringsMap.java

@@ -420,11 +420,7 @@ public List<String> get(Object key) {
         String lcKey = key.toString().toLowerCase(Locale.ENGLISH);
         String realKey = keyLookup.get(lcKey);
 
-        if (realKey == null) {
-            return null;
-        } else {
-            return values.get(realKey);
-        }
+        return realKey != null ? values.get(realKey) : Collections.<String> emptyList();
     }
 
     @Override