Generalize the logic to disallow nested objects and arrays as keys in objects.

Fixes #771.

src/main/java/org/json/JSONTokener.java

@@ -402,12 +402,7 @@ public class JSONTokener {
      */
     public Object nextValue() throws JSONException {
         char c = this.nextClean();
-        String string;
-
         switch (c) {
-        case '"':
-        case '\'':
-            return this.nextString(c);
         case '{':
             this.back();
             try {
@@ -423,6 +418,21 @@ public class JSONTokener {
                 throw new JSONException("JSON Array or Object depth too large to process.", e);
             }
         }
+        return nextSimpleValue(c);
+    }
+
+    Object nextSimpleValue(char c) {
+        String string;
+
+        switch (c) {
+        case '"':
+        case '\'':
+            return this.nextString(c);
+        case '{':
+            throw syntaxError("Nested object not expected here.");
+        case '[':
+            throw syntaxError("Nested array not expected here.");
+        }
 
         /*
          * Handle unquoted text. This could be the values true, false, or