Fix for flask-restful#873.

Use `flask.json` instead of `json` standard library as the default JSON encoder.

Overriding behavior remains, but this adds the ability for `flask_restful` to pick up the encoder defined for the app, or the api blueprint.

```python
from flask_restful import Api
from flask import Blueprint
from flask import json

class CustomJSONEncoder(json.JSONEncoder):
    def default(self, obj):
        return 'custom_flask_encoder'

blueprint = Blueprint('test', __name__)
blueprint.json_encoder = CustomJSONEncoder
api = flask_restful.Api(blueprint)
app = Flask(__name__)
app.register_blueprint(blueprint, url_prefix='/blueprint')
```

or

```python
app = Flask(__name__)
app.json_encoder = CustomJSONEncoder
api = flask_restful.Api(app)
```