Proof-of-concept: web-api-extension silent integration

For those who use [web-api-extension](https://github.com/Behat/WebApiExtension) it's not so easy to use `json_spec`. @tyx provided his proof-of-concept to integrate `WebApiContext` with `json context`: https://github.com/tyx/WebApiExtension/commit/58d832a2be924b418ca2c188c57d340c6877b8d1 but i don't like the idea of extending context to use it with another.

Solution: use wrapped Guzzle/Client to get last response.