Renewing a subscription causes notify to fail

[rtyle]

Running a subscribing & resubscribing (non peer-upnp) UPnP client against your binary-light-device causes it to fail to notify after the first resubscription.

http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.1.pdf

4.1.3 Renewing a subscription with SUBSCRIBE with SID
...
CALLBACK
(No CALLBACK header field is used to renew an event subscription.)

Each such resubscription is handled by peer-upnp as a new subscription, generating a new SID with no callbacks (as "no CALLBACK header field is used to renew"). Eventually the service is flooded with subscriptions - none of which would work because there is no callback. Actually, when notifying the first such one, the code throws an exception because it doesn't expect no callbacks.

[rtyle]

pull request #23