Use merging of event properties in async target wrapper to fix empty collection issue

The empty 'properties' collection (issue #331) was fixed by copying the properties from the original object stored in the parameters. However, the async target wrapper 'Write' is overwritten such that the merge method is never called, causing the same issue when using <targets async="true"> configuration. The solution is to call the merge operation in the overridden 'Write' method.

I ran into this issue and was able to produce a solution that seems to work for us, but we have not used NLog extensively yet, so please take a look if this fix is any good.

Thanks!