fix usage of defaultHttpCheck

The following commit inverted the logic concerning defaultHttpCheck 

https://github.com/gatling/gatling/commit/2f2753f09dc59ed2cf4c42660027f8fb90cac03c#diff-97f347b96182fceee3597f9d3c0c8a5dR110

Took me some time to spot it after most of my tests started failing with 2.1.0