Implemented matches_response on several PDUs. Added some test cases.

include/dhcpv6.h

@@ -875,6 +875,15 @@ public:
      */
     uint32_t header_size() const;
     
+        /** 
+     * \brief Check wether ptr points to a valid response for this PDU.
+     *
+     * \sa PDU::matches_response
+     * \param ptr The pointer to the buffer.
+     * \param total_sz The size of the buffer.
+     */
+    bool matches_response(uint8_t *ptr, uint32_t total_sz);
+    
     /**
      * \brief Getter for the PDU's type.
      * \sa PDU::pdu_type