Matt Johnson-Pint

Actually, related to #1492

I can reproduce it with latest 3.20.1, though with some minor changes: ``` o.Dsn = "https://[email protected]/5428537"; o.Debug = true; o.CacheDirectoryPath = "bin/temp"; ``` ``` Error: Sentry rejected the envelope 825ebfa763dd420a99243523a0165d74....

It would appear that the null entries are removed from the context section of the payload when caching is enabled, and the length header is not updated. We should ensure...

On further thought - Null contexts don't make much sense. Context is extra information at the time of the event, such as the OS name or the amount of memory...

Also I'll just add that it would have been a better design to *encapsulate* the `ConcurrentDictionary` rather than extending from it. `Contexts : IDictionary` would have allowed us to filter...

Before waiting for 4.0.0, we should also validate whether scoped data (ex: tags) makes its way through to transactions at all. If not, that's a p1 bug to solve now.

Blocked on #190. Keeping this open for followup after that is done.

So, we should send only `route` when we have it, but otherwise send `url` and don't send "unknown route" ever.

I think we can do this without waiting for 4.x, but we should note the behavior change in the changelog.

Note, this change will be visible to almost everyone that uses `.UseSentryTracing()` in ASP.Net Core for performance monitoring, as it will stop grouping 404s in the same way.