CreateClient/AddTaskActivitiesFromInterface with non-Task methods

Consider the following interface and implementation:
```
public interface IClient
{
IEnumerable GetUsers();
}

public class Client : IClient
{
public IEnumerable GetUsers()
{
return new[] { "user1", "user2" };
}
}
```

When using AddTaskActivitiesFromInterface like so:
```
taskHub.AddTaskActivitiesFromInterface(new Client());
```
and then creating a client within an orchestration:
```
var client = context.CreateClient();
var users = client.GetUsers();
foreach (var user in users)
{
string greeting = await context.ScheduleTask(typeof(SendGreetingTask), user);
}
```

This will result in an error since the returned value is not an `IEnumerable`. It appears that the actual result is `System.Threading.Tasks.Task`1[System.String]` with an underlying "ActLike_IEnumerable" type.

From the looks of it, the root cause is coming from `ScheduleProxy.TryInvokeMember`, which assumes that the returned type is of type `Task`, but "swallows" a `IEnumerable` in the same way.