HTTP: Response checks

Naive implementation of #2072
Only assuming text for `text/html` Content-Type is definitively not enough. What about css, json, xhtml, xml, etc...?
We could make [HttpHelper.isTxt](https://github.com/gatling/gatling/blob/master/gatling-http/src/main/scala/io/gatling/http/util/HttpHelper.scala#L90) more generic.