fix issue with forkjoin

[pjfanning]

relates to #485

The method handle seemed to work in my testing but today, I am seeing issues.

This change fixes the issues I have seen.

This was the exception I got and that no longer happens with this PR change.

[error] java.lang.invoke.WrongMethodTypeException: expected (int,ForkJoinWorkerThreadFactory,int,UncaughtExceptionHandler,boolean)PekkoJdk9ForkJoinPool but found (int,ForkJoinWorkerThreadFactory,int,UncaughtExceptionHandler,boolean)Object
[error] 	at java.base/java.lang.invoke.Invokers.newWrongMethodTypeException(Invokers.java:476)
[error] 	at java.base/java.lang.invoke.Invokers.checkExactType(Invokers.java:485)
[error] 	at org.apache.pekko.dispatch.ForkJoinExecutorConfigurator$ForkJoinExecutorServiceFactory.createExecutorService(ForkJoinExecutorConfigurator.scala:122)
[error] 	at org.apache.pekko.dispatch.Dispatcher$LazyExecutorServiceDelegate.executor$lzycompute(Dispatcher.scala:54)

[He-Pin]

Lgtm

[He-Pin]

Great, as the invoke will do type conversations when type not match exactly

[pjfanning]

Great, as the invoke will do type conversations when type not match exactly

I rarely find that invokeExact actually works. I'm finding in most cases that I need to use invoke anyway. I'm talking about other cases where I use MethodHandles, not necessarily in Pekko.