Support For AspNetSynchronizationContext

When the new `AspNetSynchronizationContext` was introduced it became and anti-pattern to use `PreSendRequestHeaders` from a managed module.

You could control which synchronization context an app used with an app setting:

```xml
<add key="aspnet:UseTaskFriendlySynchronizationContext" value="false" />
```
Instead of forcing users to use that setting, this pull request fixes #502 by removing our usage of `PreSendRequestHeaders`.

The ASP.NET team has [documented this](http://www.asp.net/aspnet/overview/web-development-best-practices/what-not-to-do-in-aspnet,-and-what-to-do-instead#presend) stating:

> You can use the `PreSendRequestHeaders` and `PreSendRequestContext` events with native IIS modules, but do not use them with managed modules that implement `IHttpModule`. **Setting these properties can cause issues with asynchronous requests.**

---

At this point I'm just looking to get "Ship It" squirrel or two. :shipit: