fix(calendar.js): UI calendar inside collapsed accordion rendering issue (#72)

Hi guys,

I added a new fix for https://github.com/angular-ui/ui-calendar/issues/72.

It is based on Paul's solution: https://github.com/eucaos/ui-calendar/commit/f42ef9eaab682ccf79357e6245073d4404ec1903

I would be really happy :) to have this on the main branch.

Thank you,
Costin