Inconsistent platform handling of trailing slashes in Database.Copy

See https://github.com/cbl-dart/cbl-dart/issues/444 for original discussion

Basically, if there is no trailing slash in the argument to Database.copy the operation will fail with a cryptic message on some platforms (noted here is Android, but likely Linux desktop and Windows as well).

The Dart author is going to implement a workaround, and this is otherwise able to be mitigated outside the library.