Eliminate default member values from route query

Generating a route from a request DTO currently produces redundant
parameters for value types that are equal to their default value.

This change modifies RouteMember so that it only returns a non-null value to be
contributed to the route if the current value is not equal to that type's
default value.

So previously:

```c#
public enum GenderKey
{
    None = 0,
    Male,
    Female
}

[Route("/clients/{Id}", "GET")]
public class ClientResource
{
    public int Id { get; set; }
    public GenderKey Gender { get; set; }
}
```

Might have generated a request like:
`GET /clients/25?gender=None`

With this change it will now generate:
`GET /clients/25`