Read-Only functionality for HTTP::Headers

[repomaa]

As soon as the response body receives a write call, the response headers are written to the socket. It says as much in the docs.

This is a suggestion that, in my opinion, would save a lot of devs from headache:

As soon as the response body is written the headers are set into a read-only mode, after which setting headers will raise exceptions.

I had a hard time debugging why it looks like i've set the correct response headers but none of them is sent to the client. This is increasingly hard when using web frameworks that handle the request life cycle for you.

[rdp]

Seems well received, PR encouraged! :)