Keep parent resource id as int instead of str for nested post

[Praseetha-KR]

Current behaviour:

POST on nested resources creates a resource with parent resource id value as string.

eg:

POST /posts/1/comments
{
    "body": "some comment"
}
---
{
    "body": "some comment",
    "createdAt": 1555443321417,
    "postId": "1", // <string>
    "id": 1
}

_embed doesn't pick up resources where corresponding parent resource id is string:

GET /posts/1?_embed=comments
---
{
    "id": 1,
    "title": "json-server", 
    "author": "typicode",
    "comments": [] // empty!!!
}

---

Behaviour after the fix:

This PR casts the parent resource id to int on nested resource POST request body.

POST /posts/1/comments
{
    "body": "some comment"
}
---
{
    "body": "some comment",
    "createdAt": 1555443321417,
    "postId": 1, // <int>
    "id": 1
}

Which in turn fixes _embed's response:

GET /posts/1?_embed=comments
---
{
    "id": 1,
    "title": "json-server", 
    "author": "typicode",
    "comments": [ // not empty
        {
            "body": "some comment",
            "createdAt": 1555443321417,
            "postId": 1,
            "id": 1
        }
    ]
}

[b4dnewz]

Also addressed in #931 with correct handling of string ids