Fixed web IO integration tests (GitHub HTML changed).

The following tests were failing:

```
CsQuery.Tests.Core.WebIO.GetHtml
CsQuery.Tests.Core.WebIO.GetHtmlAsyncPromisesFailed
```

The following test never completed. The infinite spin wait at the bottom is a problem...

```
CsQuery.Tests.Core.WebIO.GetHtmlAsyncPromises
```
All three problems were due to the recent change in GitHub's HTML.