RequestContextMappings should ignore private and instance fields

RequestContextMappings inspects the user defined class passed as a constructor parameter, but it checks all fields, instead of just the public static ones (at least that's what I understand it should do).

In Java 8 the issue is covered by catching an IllegalAccessException when accessing the field value, but in Java 11 this throws a NullPointerException for non-static fields.

The proposed solution would be to replace clazz.getDeclaredFields() with clazz.getFields(), to obtain only the accessible public fields, and then check them to process only the static ones.