ContributorValidationMessages compatible with UTF-8 properties

Hello,

In our (french language) project, we use bean validation with spring, we have different applications with a common library, we need to use ContributorValidationMessages.properties in each application in addition of a common ValidationMessages.properties.

ContributorValidationMessages mechanism works (nearly) great without configuration, our messages was resolved, but accented characters are not properly loaded. Our both ContributorValidationMessages.properties and ValidationMessages.properties are in UTF-8, no problem with accented characters in the ValidationMessages.properties.

After debug step by step the Hibernate Validation framework, I detected the loading between two properties files are different in method org.hibernate.validator.resourceloading.PlatformResourceBundleLocator.loadBundle.
We can see ValidationMessages (with false aggregate parameter) is loaded by default control in ResourceBundle.getBundle, but for ContributorValidationMessages with aggregate true, the properties is loaded by specific control org.hibernate.validator.resourceloading.PlatformResourceBundleLocator.AggregateResourceBundleControl

Technically ContributorValidationMessages are loaded by old style (before Java 9) java.util.Properties.load only compatible with ISO format, while ValidationMessages are loaded by new style (start with Java 9) java.util.PropertyResourceBundle.PropertyResourceBundle compatible with UTF-8 (default) and ISO.

I propose a fix in PR to modifiy org.hibernate.validator.resourceloading.PlatformResourceBundleLocator.AggregateResourceBundleControl

I remain at your disposal for any suggestion to improve code.
I will appreciate to contribute to the improvement of Hibernate Validation.

Best,