Fixes Compilation Errors in DefaultRouteResolverFixture in mono

Compilation fails by default on mono because DefaultRouteResolverFixture.cs does not bring Nancy.Tests into scope.  Therefore, on this line of code:

```csharp
result.Headers["Allow"].ShouldContain("GET");
```

the compiler assumed that the ShouldContain extension method was from Nancy.Testing.AssertExtensions instead of Nancy.Tests.ShouldExtensions