Created Issue: union of two select queries with navigation properties fails [972]

```
var query1 = db.CustomersA.Include(c => c.Orders).Select(c => new { ID = c.ID, Orders = c.Orders });
var query2 = db.CustomersB.Include(c => c.Orders).Select(c => new { ID = c.ID, Orders = c.Orders });
var query3 = query1.Union(query2);

Console.WriteLine(query3.ToString());
```
query3 fails to get translated with the following stack trace,

```
Unhandled Exception: System.ArgumentException: The 'Distinct' operation cannot b
e applied to the collection ResultType of the specified argument.
Parameter name: argument
at System.Data.Common.CommandTrees.ExpressionBuilder.Internal.ArgumentValidat
ion.ValidateDistinct(DbExpression argument)
at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.UnionTr
anslator.TranslateBinary(ExpressionConverter parent, DbExpression left, DbExpres
sion right)
at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.BinaryS
equenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpressi
on call)
at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.Sequenc
eMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression cal
l, SequenceMethod sequenceMethod)
at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTr
anslate(ExpressionConverter parent, MethodCallExpression linq)
at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(
ExpressionConverter parent, Expression linq)
at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expressi
on linq)
at System.Data.Objects.ELinq.ExpressionConverter.Convert()
at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forM
ergeOption)
at System.Data.Objects.ObjectQuery.ToTraceString()
at System.Data.Entity.Internal.Linq.InternalQuery`1.ToString()
at System.Data.Entity.Infrastructure.DbQuery`1.ToString()
at SOTests.Program.Main(String[] args) in c:\Users\ranadimi\Documents\Visual
Studio 2012\Projects\SOTests\SOTests\Program.cs:line 39
```

To get that working, I have to move the navigation properties out of the union and fetch them later like this,

```
var query1 = db.CustomersA.Include(c => c.Orders).Select(c => new { ID = c.ID });
var query2 = db.CustomersB.Include(c => c.Orders).Select(c => new { ID = c.ID });
var query3 = query1.Union(query2);
var query4 = query3.Select(q => new { ID = q.ID, Orders = db.Orders.Where(o => o.CustomerA.ID == q.ID || o.CustomerB.ID == q.ID) });

Console.WriteLine(query4.ToString());
```

Models should be self explanatory, but in case,
```
public class Customer
{
public int ID { get; set; }

public string Name { get; set; }

public virtual ICollection<Order> Orders { get; set; }
}

public class Order
{
public int ID { get; set; }

public CustomerA CustomerA { get; set; }

public CustomerB CustomerB { get; set; }
}

public class CustomerA
{
public int ID { get; set; }

public string Name { get; set; }

public virtual ICollection<Order> Orders { get; set; }
}

public class CustomerB
{
public int ID { get; set; }

public string Name { get; set; }

public virtual ICollection<Order> Orders { get; set; }
}

public class CustomersContext : DbContext
{
public DbSet<CustomerA> CustomersA { get; set; }

public DbSet<CustomerB> CustomersB { get; set; }

public DbSet<Order> Orders { get; set; }
}
```

Note: tried it with 6.0.0-alpha3-20320 with no success.