This patch fixes the entity URI in XML documents to contain the sip: part.

Without it, Polycom phones do not reset the state correctly
when receiving full state notifications.

Polycom phones indeed require the entity to be formatted exactly the same
way for all notifications it receives. From their point of view, sip:a@b
and a@b are different URIs.
Merge pull request #122 from dsandras/ds-fix-dialog-info-entity

This patch fixes the entity URI in XML documents to contain "sip:".