Edited Task: Add more advanced unit tests for TraceBeginEnd* extension methods. [1443]

Since there are a lot of changes caused by fixing [issue1201](https://aspnetwebstack.codeplex.com/workitem/1201) but the time is limited, I file this issue to add more complicated unit tests and do cleanup later. These tests are

1. Creation of expected Message (current tests overwrite the Message in `errorTrace` Action).
2. More complicated `HttpResponseException` extraction e.g. buried down a level or two and multiple `HttpResponseException` objects in an `AggregateException`.
3. Combine 1 & 2 to confirm at least one Message creation that actually joins two `exception.Message` strings.