Fix Baggage header parsing in ASP.NET (Framework)

[kuhnroyal]

This uses the wrong header name when parsing the baggage.

[mattjohnsonpint]

For everyone's awareness, this bug fix applies to ASP.NET (framework), not ASP.NET Core.

The baggage header is used by Sentry for supplying dynamic sampling context during distributing tracing. So if you are using ASP.NET (framework) to server APIs, and those APIs are being called by other applications that are instrumented with Sentry, then those applications would pass the baggage header containing dynamic sampling context. This bug fix is important for that scenario.

If you're just rendering HTML content directly (such as with MVC views, etc.), then this doesn't apply.

@kuhnroyal - Thanks for finding and fixing this!