Added tests to exercise intercepting an async method

[JSkimming]

I'm re-raising this PR originally raised as #337 to present the problem.

I'm going to then merge the changes from @brunoblank in #428 and re-factor given the breaking changes to see if it fixes the issue (spoiler alert, it does on my machine 😃)

[JSkimming]

As shown by the passing builds, the changes introduced by PR #428 address the problem I outlined in issue #145 specifically this comment.

[stakx]

@JSkimming - Apologies for the long silence from the Castle team. I've been wanting to take a careful look at your PR and the associated issue for a long time, but have been very preoccupied with work. This is just a small ping to show that this hasn't been forgotten. I'm planning to get back to work here at this repo soon... but it might take me another few weeks.

[stakx]

@JSkimming - Thank you for adding a unit test to demonstrate a problem that #428 would solve. 👍

However we now have two PRs that propose the same basic change to DynamicProxy's API; I've posted a review comment over in #428 that would also apply here. In order to not spread discussion over too many separate pages and end up with excessive cross-referencing, I propose that we discuss this change over in #428 or, if we stray too far from it, go back to the issue (#145). Is this OK with you?

[JSkimming]

@stakx of course makes perfect sense.