sdp: Always use ActPass when sending an offer

[bekriebel]

When I was investigating a different issue, I came across this discrepancy. I don't have a real-world issue that this is affecting, but I wanted to open the PR in case you want to align with the RFC.

The text from the RFC:

5.5. Modifying the Session
When an offerer sends a subsequent offer, if the offerer wants to establish a new DTLS association, the offerer MUST insert an SDP "setup" attribute [RFC4145] with an "actpass" attribute value, as well as or more SDP "fingerprint" attributes according to the procedures in [RFC8122]. In addition, the offerer MUST insert in the offer an SDP "tls-id" attribute with a new unique attribute value.

When an offerer sends a subsequent offer and does not want to establish a new DTLS association, if a previously established DTLS association exists, the offerer MUST insert in the offer an SDP "setup" attribute with an "actpass" attribute value, and one or more SDP "fingerprint" attributes with attribute values that do not change the previously sent fingerprint set. In addition, the offerer MUST insert an SDP "tls-id" attribute with the previously assigned attribute value in the offer.

NOTE: When a new DTLS association is being established, each endpoint needs to be prepared to receive data on both the new and old DTLS associations as long as both are alive.