defaultViewDate modify my input variable

[loverg-c]

Expected behaviour

When i pass a date object as an input for the defaultViewDate, this variable should not be modified when i'm changing the viewDate by switching my view manually

Actual behaviour

My input variable is changed when i'm switching the view mode manually

Step to reproduce

Given I have a datepicker
And I pass a Date variable as defaultViewDate option,
When I open the datepicker
And I click on the month view
And I click on another month than my defaultViewDate
Then my date variable has been changed

Datepicker version used

1.9.0

Example code

https://jsfiddle.net/g7r0ow4y/

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I think its maybe related to this commit 8c99457 , because other case use a copy of the startDate (by doing a new Date), but not on this else.

(Maybe i'm wrong, i didn't investigate so much and i'm not used to js)