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

@@ -413,9 +413,8 @@ public String getJoinedValue(String key, String delimiter) {
 
     @Override
     public List<String> get(Object key) {
-        if (key == null) {
-            return null;
-        }
+        if (key == null)
+            return Collections.emptyList();
 
         String lcKey = key.toString().toLowerCase(Locale.ENGLISH);
         String realKey = keyLookup.get(lcKey);