More lenient handling of non-Publisher values for subscriptions

[rstoyanchev]

Although not common, it should be possible for a subscription handler to return a concrete value. The same is possible with just GraphQL Java, and we should handle it more leniently, essentially treating it as a Publisher of one.

There was a related recent report #1143 to allow a DataFetcherResult with an error. We should expand on that and make it more general to support any subscription return value.