FakeS3::FileStore#copy_object should not remove the src object

When using the `copy_object` method, the src object is removed.

Here is a script to show the issue using the aws-sdk gem:

```ruby
s3 = AWS::S3.new(
  :access_key_id => '123',
  :secret_access_key => 'abc',
  :s3_endpoint => 'localhost',
  :s3_port => 8888,
  :use_ssl => false
)

b = s3.buckets["test"]
b.objects["a"].write("foo!")

o = b.objects["a"]
o.copy_to("b")

puts b.objects.size # 1 which is wrong, should be 2
```