Make Roles/Users virtual

Fixes https://github.com/aspnet/Identity/issues/514

src/Microsoft.AspNet.Identity.EntityFramework/RoleStore.cs

@@ -285,7 +285,7 @@ namespace Microsoft.AspNet.Identity.EntityFramework
             }
         }
 
-        public IQueryable<TRole> Roles
+        public virtual IQueryable<TRole> Roles
         {
             get { return Context.Set<TRole>(); }
         }

src/Microsoft.AspNet.Identity.EntityFramework/UserStore.cs

@@ -238,7 +238,7 @@ namespace Microsoft.AspNet.Identity.EntityFramework
             return Users.FirstOrDefaultAsync(u => u.NormalizedUserName == normalizedUserName, cancellationToken);
         }
 
-        public IQueryable<TUser> Users
+        public virtual IQueryable<TUser> Users
         {
             get { return Context.Set<TUser>(); }
         }