Clients now receive HTTP status codes like 200, 404, etc.
Note that a 404 with content is still considered a "successful"
download from ProtocolServer's perspective. It's up to the client
to interpret the status code.
I'm not sure if this is the best API, but it'll work for now.
You can now pass a dictionary of request headers when starting a new
download in ProtocolServer.
The HTTP and HTTPS protocol will include the headers in their requests.
