Edited Issue: Hard to support customize service document [577]

If user wants to implement his own metadata controller, it will be hard for him to get the service document.

1. There is no virtual method in ODataMetadataController to let user to override EdmModel

2. If user implements controller from ApiController, it's hard to build service document because we has no public API to get entity set url.

Here is the product code:
public HttpResponseMessage GetServiceDocument()
{
IEdmModel model;
MediaTypeFormatter odataFormatter = GetFormatter(typeof(ODataWorkspace), out model);

ODataWorkspace workspace = new ODataWorkspace();
IEdmEntityContainer container = model.EntityContainers().Single();
IEnumerable<IEdmEntitySet> entitysets = container.EntitySets();

IEnumerable<ODataResourceCollectionInfo> collections = entitysets.Select(e => GetODataResourceCollectionInfo(model.GetEntitySetUrl(e).ToString(), e.Name));
workspace.Collections = collections;

// This controller requires and supports only the OData formatter.
return Request.CreateResponse(HttpStatusCode.OK, workspace, odataFormatter);
}

private static ODataResourceCollectionInfo GetODataResourceCollectionInfo(string url, string name)
{
ODataResourceCollectionInfo info = new ODataResourceCollectionInfo
{
Url = new Uri(url, UriKind.Relative)
};

info.SetAnnotation<AtomResourceCollectionMetadata>(new AtomResourceCollectionMetadata { Title = name });

return info;
}

The GetEntitySetUrl extension method is internal.