[BUG] create helper function ignores the id property when it's equal to 0.

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**Description** :
Create call on a loopback model returns a property id as a response, in case where the property id starts from 0 it is removed from response. This is because the create function( dao.js line no 378) which gets the id as a callback on the create call in a loopback model, does not accepts 0 as an id.

Code snippet from dao.js
if id is 0 it is skipping the if block
```
function createCallback(err, id, rev) {
if (id) {
obj.__data[_idName] = id;
defineReadonlyProp(obj, _idName, id);
}
````
We have a scenario where the id starts from 0, and this is causing the id property for our 0th element to be removed from the response.

Steps to reproduce :
* Model which has a property id which starts from 0.
* Call the api to create which will have the id (starting from 0) in response.
* Response will be missing the id property for the first call where id is 0, and it'll add the id property in response starting from id as 1 onwards.

Current Response Behaviour :
id is removed when its value is equal to 0

```
{
"property1" : "property1",
"property2" : "property2"
}

```

Expected Results :

```
{
"id" : 0
"property1" : "property1",
"property2" : "property2"
}

```

Start at the createCallback function in dao.js around line 378 and trace how the callback id is assigned to the created object. Reproduce a create call with an id of 0, then add coverage for the expected response and verify that ids of 0 and greater are preserved.

Written by the indexing model from the issue text.