Replace indexOf() with contains()

[arturobernalg]

jsparrow -->

'Most checks against an indexOf value compare it with -1 because 0 is a valid index. Any checks which look for values >0 ignore the first element, which is likely a bug. If the intent is merely to check inclusion of a value in a String or a List, the contains method is better suited to express this intent. Clearer intent means better readability.'

[coveralls]

Coverage remained the same at 78.278% when pulling 7f02be4 on arturobernalg:feature/use_contains into 5165528 on apache:master.