Make sure a cloned tempfile retains its tempfile magic.

That is, don't deleted the temp file until all instances of the
Path::Tiny object are destroyed.
Path::Tiny objects are immutable, no need to clone them.

Also use blessed() to check if a thing is a Path::Tiny object.  This is
twice as fast as doing the eval { $path->isa(__PACKAGE__) }.  Though
it violates good OO practice, Path::Tiny is designed not to be
subclassed.  Don't ask me, man, I just patch here.