Proper 'Connection' header handling compatible with HTTP/1.0 and HTTP/1.1

[iaintshine]

Hi,

I hope you find this pull request useful. It includes a fix which makes puma handle the 'Connection' header properly.

In the current state, when you make an HTTP/1.1 request with the 'Connection' header set to 'close', you expect it to always respond with the 'Connection: close'. It's not the case if the server responds with a 204 status code or any other code without a body. The response won't include the 'Connection' header at all. This behavior can cause potential issues, such as writing to a socket that's already closed if some proxy server rewrites the 'Connection' header to 'close' and a client expects the connection to last for more than one request. Even though the HTTP/1.1 Message and Routing RFC - Persistence states that connection can be closed at any time, with or without intention it's always nice to inform a client about the server intention to close the connection.

The pull request changes the current state to behave in the following way:

• If the received protocol is HTTP/1.1, and no 'Connection' header is present, or the header is set to 'keep-alive', the connection will persist and the header won't be present in the response
• If the received protocol is HTTP/1.1, and the 'Connection' header is present and is set to 'close', the connection will not persist and the response will include the 'Connection' header set to 'close'
• If the received protocol is HTTP/1.0, and no 'Connection' header is present, or the header is set to 'close', the connection will not persist and the header won't be present in the response
• If the received protocol is HTTP/1.0, and the 'Connection' header is present and is set to 'keep-alive', the connection will persist and the response will include the 'Connection' header set to 'keep-alive'

Hope that make sense.

Kind Regards

[evanphx]

Thanks! Great work!