Fix bug in directory detection logic for GCS

[nnutter]

If a file exists that is a superset of the filename, for example, "file1" and "file12", then this logic would erroneously report the subset path as a directory due to the lack of the trailing delimiter on the prefix.

I don't think the mocks used in unit tests can reproduce this behavior but an example of the behavior using the actual GCS client,

    client := ...
    bucketName := ...

    ctx := context.Background()

    it := client.Bucket(bucketName).Objects(ctx, &storage.Query{Delimiter: "/", Prefix: "testing/file1", Versions: false})
    _, err = it.Next()
    assert.ErrorIs(t, err, iterator.Done)

    object := client.Bucket(bucketName).Object("testing/file12")
    w := object.NewWriter(ctx)
    _, err = w.Write([]byte("test"))
    assert.NoError(t, err)
    err = w.Close()
    assert.NoError(t, err)
    t.Cleanup(func() {
        err := object.Delete(ctx)
        assert.NoError(t, err)
    })

    it = client.Bucket(bucketName).Objects(ctx, &storage.Query{Delimiter: "/", Prefix: "testing/file1/", Versions: false})
    _, err = it.Next()
    assert.ErrorIs(t, err, iterator.Done)

    it = client.Bucket(bucketName).Objects(ctx, &storage.Query{Delimiter: "/", Prefix: "testing/file1", Versions: false})
    _, err = it.Next()
    // This will fail because "file12" is found but that does not mean "file1" is a directory.
    assert.ErrorIs(t, err, iterator.Done)

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