Feature: Routing overload

[youknowone]

When user specifies HTTP methods to function handlers, it automatically
will be overloaded unless they duplicate.

Example:

    # This is a new route. It works as before.
    @app.route('/overload', methods=['GET'])
    async def handler1(request):
        return text('OK1')

    # This is the exiting route but a new method. They are merged and
    # work as combined. The route will serve all of GET, POST and PUT.
    @app.route('/overload', methods=['POST', 'PUT'])
    async def handler2(request):
        return text('OK2')

    # This is the existing route and PUT method is the duplicated method.
    # It raises RouteExists.
    @app.route('/overload', methods=['PUT', 'DELETE'])
    async def handler3(request):
	return text('Duplicated')

[seemethere]

Thanks! Great job @youknowone!

[youknowone]

By the way, writing this patch, I had a question:
When adding a function to route, why its default methods is not just ['GET'] but all methods?
It looks weird that users must add methods=['GET'] to limit the user access only to get method.
URL endpoints are (practically) never designed to work for all of the HTTP methods. They only work for only few method.

[seemethere]

@youknowone That's definitely something we should look into. Maybe it would be beneficial to just default it to ['GET']

[AntonDnepr]

Not tested this, but the implementation of this could cause this error as far as I can see:
#267 (comment)
maybe I'm wrong, but it looks the very similar way as HttpMethodView before.

[youknowone]

@AntonDnepr I agree. I never thought about it. I think the mechanism itself is not harmful but its naming makes confusion. How about the class name does not end with 'view' but ends with 'route' that means it is persistent? Like HTTPMethodRoute and MethodRedirectionRoute.