Notifications exceed default block size

[chrysn]

At least when no block size is negotiated, notifications produce updates exceeding the default datagram size, possibly getting lost in transmission.

Steps to reproduce:

• Sniff on loopback interface
• Make the time resource of the demo server output something large
• aiocoap-client observe coap://localhost/time

The network dumps show that while the first requests are properly block-wised, the notifications arrive in a single overly large coap message.