Case variable: "readonly" tag permits changing value after reopening case

After cancelling a case, a variable tagged as "readonly" is added again in the case file with a different value when reopening the case. A VariableViolationException is expected, as these variables are not allowed to be set more than once, but it isn't thrown, changing its value and ignoring the restriction.

Notice that currently for adding "readonly" variables when starting the case, it's not possible if the value is added through newCaseFileInstance, as when creating the case file with that variable, is considered as first assignation of the value and therefore it cannot be modified at startCase. This is also a weird behavior as makes "readonly" variables useless when starting the case.