Fix meta update in gen exchange (for meta service string updates)

- was assuming all meta changes were related to bit sets
- was returning fail, when successful

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7397 b45a01b8-16f6-495d-af2f-9b41ad6348cc

libretroshare/src/util/contentvalue.h

@@ -161,6 +161,11 @@ public:
      */
     void clear();
 
+    /*!
+     * checks if internal kv map is empty
+     */
+    bool empty() const;
+
 private:
 
     /*!