Set route set for 18x with to tag

[yeoleobun]

fix #44

According RFC 3261:

12.1.2 UAC Behavior

When a UAC receives a response that establishes a dialog, it
constructs the state of the dialog. This state MUST be maintained
for the duration of the dialog.

13.2.2.1 1xx Responses

Provisional responses for an
INVITE request can create "early dialogs". If a provisional response
has a tag in the To field, and if the dialog ID of the response does
not match an existing dialog, one is constructed using the procedures
defined in Section 12.1.2.

Early dialog should count as dialog, which created by 180/183 with to tag. Should also the route set from Record-Route header.

---

12.1.2 UAC Behavior

If no Record-Route header field is present in the
request, the route set MUST be set to the empty set.

so I remove the predication:

        if new_route_set.is_empty() {
            return;
        }