`HttpHeaderBase.Headers` requires checking contents with correct case

[andrensairr]

HTTP header names are case insensitive, and this library helpfully converts headers to upper case for easy reading. However, I think the actual case of the Headers dictionary keys is an implementation detail that is not easily discovered from the public interface. Dictionary keys must be read or checked for their existence using something like message[key.ToUpper()] or message.Headers.ContainsKey(key.ToUpper()). This is not a problem in itself, but headers like Accept-Encoding and Content-Length are often sent transmitted case, so it is not intuitive to use case sensitive dictionary searching.