SUBSCRIBE refresh requests do not contain a To tag

[josephfrazier]

According to RFC 6665 Section 4.1.2.2 (emphasis mine):

At any time before a subscription expires, the subscriber may refresh
the timer on such a subscription by sending another SUBSCRIBE request
on the same dialog as the existing subscription. The handling for
such a request is the same as for the initial creation of a
subscription except as described below.

However, a SIP.js Subscription refreshes itself by calling its subscribe method, as seen on lines 94 and 199 of Subscription.js. Since the subscribe method simply (re)sends the initial SUBSCRIBE request, it does not contain a To tag, but it should according to RFC 3261 Section 12.2.1.1 (emphasis mine):

The URI in the To field of the request MUST be set to the remote URI
from the dialog state. The tag in the To header field of the request
MUST be set to the remote tag of the dialog ID. The From URI of the
request MUST be set to the local URI from the dialog state. The tag
in the From header field of the request MUST be set to the local tag
of the dialog ID. If the value of the remote or local tags is null,
the tag parameter MUST be omitted from the To or From header fields,
respectively.

This can be demonstrated by using code from #86 to create a UA with traceSip:true, calling UA#subscribe with a small value for the expires option, and observing the resulting SUBSCRIBE requests. Notice that the refresh requests actually create separate subscriptions, which explains why they have different ids in Subscription#receiveResponse.

[wakamoleguy]

Fixed in f80adf8