0.20.0

Features

• Add support for extraHeaders on Invitation.accept (#910)
• Make the interval at which re-REGISTER requests are sent configurable (UserAgentOptions.refreshFrequency) (#902)
• Add onAck delegation for Session (#911)

Fixes

• add gruu check to registerer to restore previous behavior
• publisher: add check so that it does not attempt to enter published state when already there
• fix a typo in extra headers for an uncommon failure case