Replace tests with httpbin service

[razbakov]

I suggest to use httpbin to replace www fixture and make tests more readable.

[elkangaroo]

I dont't think it is a good idea to rely on an external service for the tests.

[razbakov]

Then maybe index.php should return at least json?

[alexislefebvre]

@razbakov I don't know how httpbin was one year ago but it can now be launched as a Docker container with docker run -p 80:80 kennethreitz/httpbin so it can be launched locally, it would not be an external service.