Use source object prototype in object copy

[myitcv]

Whilst not strictly necessary for the likes of $watch to behave as expected, given that angular.copy is exposed it should honour the prototype of the source object such that, as the test ensures:

function A() {}
var a = new A();
var a_copy = angular.copy(a);
expect(a_copy instanceof A).toBe(true);

Otherwise users of angular.copy (read: at least me) end up having to create their own version of angular.copy to make this one change.

[lgalfaso]

Thanks for the PR, but it looks like this issue is a duplicate of #5063 I am closing it now but feel free to open in case there are other issues not included in the other PR

[myitcv]

@lgalfaso - apologies, I did search before posting (forget the search term I used) but clearly my search was bad... Thanks for finding the dup.

[lgalfaso]

@myitcv it is not a problem