set default to proposeStart/End to avoid Error:Typ eError: proposeStart.getInTimezone is not a function

[jaroslawp]

This should fix:

https://github.com/Ericsson/exchangecalendar/issues/532 (and: https://github.com/Ericsson/exchangecalendar/issues/579 https://github.com/Ericsson/exchangecalendar/issues/542

• which seem to be same problem ?)

[jaroslawp]

that could be better fix .... avoids proposing current date/time as new proposedStart/End

[wtfrank]

This solves an issue I've been having for a while accepting meeting invites.

Anything stopping this small fix being merged into the main branch?