Replace java 8 method putIfAbsent with java 7 compatable approach

Author: bcsorba

src/main/java/org/piwik/java/tracking/CustomVariableList.java

@@ -1,6 +1,6 @@
-/* 
+/*
  * Piwik Java Tracker
- * 
+ *
  * @link https://github.com/piwik/piwik-java-tracker
  * @license https://github.com/piwik/piwik-java-tracker/blob/master/LICENSE BSD-3 Clause
  */
@@ -31,24 +31,28 @@ void add(CustomVariable cv){
         }
         if (!found){
             int i = 1;
-            while (map.putIfAbsent(i++, cv) != null){}        
+            while (map.containsKey(i)) {
+                ++i;
+            }
+
+            map.put(i, cv);
         }
     }
-    
+
     void add(CustomVariable cv, int index){
         if (index <= 0){
             throw new IllegalArgumentException("Index must be greater than 0.");
         }
         map.put(index, cv);
     }
-    
+
     CustomVariable get(int index){
         if (index <= 0){
             throw new IllegalArgumentException("Index must be greater than 0.");
         }
         return map.get(index);
     }
-    
+
     String get(String key){
         Iterator<Entry<Integer, CustomVariable>> i = map.entrySet().iterator();
         while (i.hasNext()){
@@ -59,11 +63,11 @@ String get(String key){
         }
         return null;
     }
-    
+
     void remove(int index){
         map.remove(index);
     }
-    
+
     void remove(String key){
         Iterator<Entry<Integer, CustomVariable>> i = map.entrySet().iterator();
         while (i.hasNext()){
@@ -73,22 +77,22 @@ void remove(String key){
             }
         }
     }
-    
+
     boolean isEmpty(){
         return map.isEmpty();
     }
-        
+
     @Override
     public String toString(){
         JsonObjectBuilder ob = Json.createObjectBuilder();
-        
+
         for (Entry<Integer, CustomVariable> entry : map.entrySet()){
             JsonArrayBuilder ab = Json.createArrayBuilder();
             ab.add(entry.getValue().getKey());
             ab.add(entry.getValue().getValue());
             ob.add(entry.getKey().toString(), ab);
         }
-        
+
         return ob.build().toString();
     }
 }