Pull request: https://github.com/jenkinsci/parameterized-trigger-plugin/pull/94

ParameterDefinition types which extend StringParameterDefinition still need to be converted, when possible, in order to ensure correct handling of any additional logic implemented by the sub-type.

The code change is simply to use a more exacting check then the instanceof keyword because any parameter types which sub-class StringParameterDefinition in order to avoid re-implemeting its behaviour are not currently converted when they could be.