Added assertion

Documented existing assertions in EndObject
Added new assertion in EndObject to catch error condition where objects
are ended with a key but no matching value.

include/rapidjson/writer.h

@@ -221,8 +221,9 @@ public:
 
     bool EndObject(SizeType memberCount = 0) {
         (void)memberCount;
-        RAPIDJSON_ASSERT(level_stack_.GetSize() >= sizeof(Level));
-        RAPIDJSON_ASSERT(!level_stack_.template Top<Level>()->inArray);
+        RAPIDJSON_ASSERT(level_stack_.GetSize() >= sizeof(Level)); // not inside an Object
+        RAPIDJSON_ASSERT(!level_stack_.template Top<Level>()->inArray); // currently inside an Array, not Object
+        RAPIDJSON_ASSERT(0 == level_stack_.template Top<Level>()->valueCount % 2); // Object has a Key without a Value
         level_stack_.template Pop<Level>(1);
         return EndValue(WriteEndObject());
     }