When a client sends a CoAP request with no block2 size,
the default one would be set to REST_MAX_CHUNK_SIZE.
However, this is not guaranteed to be a power of 2.
This can lead to clients receiving a bigger payload than expected as
part of the header, and ending up with duplicated content.
Setting the default to COAP_MAX_BLOCK_SIZE,
which is guaranteed to be a power of 2, fixes this.