Java Event is missing an implementation of `validate_value`

https://github.com/elastic/logstash/blob/master/logstash-core-event-java/src/main/java/com/logstash/ext/JrubyEventExtLibrary.java#L308

``` java
@JRubyMethod(name = "validate_value", required = 1, meta = true)
public static IRubyObject ruby_validate_value(ThreadContext context, IRubyObject recv, IRubyObject value)
{
// TODO: add UTF-8 validation
return value;
}
```

Is this even still necessary? @colinsurprenant

Start with logstash-core-event-java/src/main/java/com/logstash/ext/JrubyEventExtLibrary.java at the validate_value method around line 308, then inspect its callers and related event validation behavior. Determine whether UTF-8 validation is still required; done means resolving that question and, if required, adding the validation with coverage for the affected behavior.

Written by the indexing model from the issue text.