MINOR: http: turn default error files to HTTP/1.1

MINOR: http: turn default error files to HTTP/1.1

For quite a long time we've been saying that the default error files
should produce HTTP/1.1 responses and since it's of low importance, it
always gets forgotten.

So here it finally comes. Each status code now properly contains a
content-length header so that the output is clean and doesn't force
upstream proxies to switch to chunked encoding or to close the connection
immediately after the response, which is particularly annoying for 401
or 407 for example. It's worth noting that the 3xx codes had already
been turned to HTTP/1.1.

This patch will obviously not change anything for user-provided error files.