net: coap_client: Don't send RST on unrecognized ACK response

According to CoAP RFC (7252, ch. 4.2), Reset cannot be sent in a
response for an ACK reply, rejected ACKs should be silently ignored
instead. Therefore, in case an unrecognized response is received, verify
the response type before sending Reset.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>