Fix check already attached document

In AttachDocument, [ErrDocumentAlreadyAttached] error is returned if
there is a document in the current client field. However, AttachDocument
also need to check the document attachment status to determine whether to
return an error or not.

client/client_test.go

@@ -124,6 +124,14 @@ func TestClientAndDocument(t *testing.T) {
 		err = c1.Detach(ctx, doc)
 		assert.NoError(t, err)
 		assert.False(t, doc.IsAttached())
+
+		err = c1.Attach(ctx, doc)
+		assert.NoError(t, err)
+		assert.True(t, doc.IsAttached())
+
+		err = c1.Detach(ctx, doc)
+		assert.NoError(t, err)
+		assert.False(t, doc.IsAttached())
 	})
 
 	t.Run("causal nested array test", func(t *testing.T) {

yorkie/types/client_info.go

@@ -68,7 +68,7 @@ func (i *ClientInfo) AttachDocument(docID primitive.ObjectID) error {
 
 	hexDocID := docID.Hex()
 
-	if i.hasDocument(hexDocID) {
+	if i.hasDocument(hexDocID) && i.Documents[hexDocID].Status == DocumentAttached {
 		return ErrDocumentAlreadyAttached
 	}