JJavaName pluralisation incorrect for checksum

[petergeneric]

The rules defined for JJavaName https://github.com/javaee/jaxb-v2/blob/769b89aef9231d4f8d140b91b03c3afa6aaa1c86/jaxb-ri/codemodel/codemodel/src/main/java/com/sun/codemodel/JJavaName.java#L234 are converting all words ending in um -> a, but this fails for an obvious usecase, "checksum" which gets converted to "checksa" rather than "checksums"