block2 request

coap.js now can request for a resource that return in blockwise(2).

Tested with coap.request('coap://coap.me/.well-known/core').

Todo
- block2 for observe
- block1

The consequence request do not have token, but block2 option; hence, I try not to change the original request (except packet id) by making a clone, which will take effect in case of observation with blockwise.