Strongly typed headers are great, but for the love of God please give us a back door. And I don't mean just a dictionary for every "unknown" header, but we should be able to override every "known" header as well.

I'll give you a real-world example. Mono. HttpWebRequest used to store the Content-Size value as an int32. What about files larger than 4gb? Oops...

That was a big problem for us. Thankfully Mono is open source, we were able to peruse the code and come up with some magic incantations using reflection to work around the fact that the framework libraries would not allow us to set the Content-Size header without going through the int32 property.

Now, normally you would file that as a bug, however, it (A) took years for a version of mono to be released with that fixed this and (B) could easily have been some misunderstanding in the implementation of the HTTP spec that was overlooked. You're implementing a spec right now. Can you guarantee that the your implementation is 100% correct? Can you guarantee that the spec is 100% correct? Can you guarantee that the spec is implemented 100% correct everywhere?

No, you can't.

So, don't limit me. Help me, but do not prevent me from doing what I need to in order to do my job.