Merge pull request stleary#360 from migueltt/tokener-fix

Creating a JSONObject from a string that contains a duplicate key (any level) throws a JSONException that includes location

JSONObject.java

@@ -231,7 +231,21 @@ public JSONObject(JSONTokener x) throws JSONException {
             if (c != ':') {
                 throw x.syntaxError("Expected a ':' after a key");
             }
-            this.putOnce(key, x.nextValue());
+
+            // Use syntaxError(..) to include error location
+
+            if (key != null) {
+                // Check if key exists
+                if (this.opt(key) != null) {
+                    // key already exists
+                    throw x.syntaxError("Duplicate key \"" + key + "\"");
+                }
+                // Only add value if non-null
+                Object value = x.nextValue();
+                if (value!=null) {
+                    this.put(key, value);
+                }
+            }
 
             // Pairs are separated by ','.