Use flask.json as default encoder

[juannorris]

Fix for #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.

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

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