rcat: preserve metadata when Copy falls back to Rcat

Before this change if we copied files of unknown size, then they lost
their metadata.

This was particularly noticeable using --s3-decompress.

This change adds metadata to Rcat and RcatSized and changes Copy to
pass the metadata in when it calls Rcat for an unknown sized input.

Fixes #6546

backend/chunker/chunker_internal_test.go

@@ -759,7 +759,7 @@ func testFutureProof(t *testing.T, f *Fs) {
 
 	// Rcat must fail
 	in := io.NopCloser(bytes.NewBufferString("abc"))
-	robj, err := operations.Rcat(ctx, f, file, in, modTime)
+	robj, err := operations.Rcat(ctx, f, file, in, modTime, nil)
 	assert.Nil(t, robj)
 	assert.NotNil(t, err)
 	if err != nil {