Accept option of observes must be selectable - LWM2M implementation

The accept option tells the lwm2m client which encoding to use in a response. This could be TLV, JSON or others. There are situations (for example for buffering data) where it is necessary to select the accept option manually. TLV does not contain any timestamp information whereas JSON does. If we would transmit buffered data the timestamp of the record is needed. Hence it must be possible to select the accept option for observes in order to keep track of the timestamps. Other LWM2M server implementations are supporting this feature.