EnumValidator should avoid trying to modify protected nodes

EnumValidator is trying to "correct" inputs by converting the input node to the official name of the enum. This results a caught and discarded UnsupportedOperationException if the node is protected. Catching an exception is a poor way to do this.

Simply not doing the write if the input is the same as the official value is one way to mitigate this, but better is to wait for DMR-37 and just check if the node is protected.