DataReader remains open in some cases

[AlexUstinov]

The code below demonstrates the issue with the data reader. It looks like in some cases underlying SQL data reader remains open after enumeration blocking attempts to commit a transaction.
The code below demonstrates the issue.

  // The model
  [HierarchyRoot]
  [KeyGenerator(KeyGeneratorKind.None)]
  public class Item: Entity
  {
    [Field, Key]
    public Guid Id { get; private set; }

    public Item(Session session, Guid id)
      : base(session, id)
    { }
  }

  // The failing code
  // NOTE: There are no Item records in the table
  using (var session = Domain.OpenSession()) {
    using (var tx = session.OpenTransaction()) {
      var list = new List<Guid>();
      var sequence = Enumerable.Empty<Guid>().Union(session.Query.All<Item>()
        .Where(item => item.Id == Guid.Empty)
        .Select(item => item.Id));
      foreach (var guid in sequence) {
        list.Add(guid);
      }

      Assert.AreEqual(0, list.Count);
      tx.Complete();
    }
  }

When executed, this code throws an exception on the attempt to complete the transaction using block.

Xtensive.Orm.StorageException : SQL error occured.
SQL error details 'Type: Unknown;'
Original message 'The transaction operation cannot be performed because there are pending requests working on this transaction.'
  ----> System.Data.SqlClient.SqlException : The transaction operation cannot be performed because there are pending requests working on this transaction.