Large observe packets may cause partial buffers to be sent

If a single output buffer for an observe command exceeds 512 bytes, the library may silently drop subsequent items to be appended to the command body.

This is normally not an issue in practice as the observe buffers are typically issued on a per-key basis; and since a key is never larger than 256b, the library accidentally never runs into this bug. This only seems to be an issue with so-called "multi observe" or "multi endure" commands.