refactor(Identifiable): there is no need to make Id abstract

Author: jaredcnance

README.md

@@ -76,9 +76,7 @@ Your models should inherit `Identifiable<TId>` where `TId` is the type of the pr
 
 ```csharp
 public class Person : Identifiable<Guid>
-{
-    public override Guid Id { get; set; }
-}
+{ }
 ```
 
 #### Specifying Public Attributes
@@ -89,8 +87,6 @@ add the `AttrAttribute` and provide the outbound name.
 ```csharp
 public class Person : Identifiable<int>
 {
-    public override int Id { get; set; }
-    
     [Attr("first-name")]
     public string FirstName { get; set; }
 }
@@ -104,8 +100,6 @@ they should be labeled as virtual.
 ```csharp
 public class Person : Identifiable<int>
 {
-    public override int Id { get; set; }
-    
     [Attr("first-name")]
     public string FirstName { get; set; }
 
@@ -119,8 +113,6 @@ For example, a `TodoItem` may have an `Owner` and so the Id attribute should be
 ```csharp
 public class TodoItem : Identifiable<int>
 {
-    public override int Id { get; set; }
-    
     [Attr("description")]
     public string Description { get; set; }
 

src/JsonApiDotNetCore/Models/Identifiable.cs

@@ -2,7 +2,7 @@ namespace JsonApiDotNetCore.Models
 {
     public abstract class Identifiable<T> : IIdentifiable<T>, IIdentifiable
     {
-        public abstract T Id { get; set; }
+        public T Id { get; set; }
 
         object IIdentifiable.Id
         {

src/JsonApiDotNetCoreExample/Models/Person.cs

@@ -7,8 +7,6 @@ namespace JsonApiDotNetCoreExample.Models
 {
     public class Person : Identifiable<int>, IHasMeta
     {
-        public override int Id { get; set; }
-        
         [Attr("first-name")]
         public string FirstName { get; set; }
 

src/JsonApiDotNetCoreExample/Models/TodoItem.cs

@@ -5,7 +5,6 @@ namespace JsonApiDotNetCoreExample.Models
 {
     public class TodoItem : Identifiable<int>
     {
-        public override int Id { get; set; }
 
         [Attr("description")]
         public string Description { get; set; }

test/JsonApiDotNetCoreExampleTests/Acceptance/Spec/DocumentTests/Included.cs

@@ -13,7 +13,6 @@
 using JsonApiDotNetCoreExample.Data;
 using Bogus;
 using JsonApiDotNetCoreExample.Models;
-using System;
 
 namespace JsonApiDotNetCoreExampleTests.Acceptance.Spec.DocumentTests
 {