Send the proper headers to HTTP/1.0 in keep-alive

Seems that an HTTP/1.0 client needs to be told that the connection is
still in keep alive mode by setting the Connection header to Keep-Alive.

This makes sense because a server could deny the request and thus no
Connection header means close mode.

lib/puma/server.rb

@@ -245,9 +245,11 @@ module Puma
         allow_chunked = true
         http_version = "HTTP/1.1 "
         keep_alive = env["HTTP_CONNECTION"] != "close"
+        include_keepalive_header = false
       else
         http_version = "HTTP/1.0 "
         keep_alive = env["HTTP_CONNECTION"] == "Keep-Alive"
+        include_keepalive_header = keep_alive
       end
 
       chunked = false
@@ -294,7 +296,11 @@ module Puma
           end
         end
 
-        client.write "Connection: close\r\n" unless keep_alive
+        if include_keepalive_header
+          client.write "Connection: Keep-Alive\r\n"
+        elsif !keep_alive
+          client.write "Connection: close\r\n"
+        end
 
         if content_length
           client.write "Content-Length: #{content_length}\r\n"

test/test_persistent.rb

@@ -123,7 +123,7 @@ class TestPersistent < Test::Unit::TestCase
     @client << @keep_request
     sz = @body[0].size.to_s
 
-    assert_equal "HTTP/1.0 200 OK\r\nX-Header: Works\r\nContent-Length: #{sz}\r\n\r\n", lines(4)
+    assert_equal "HTTP/1.0 200 OK\r\nX-Header: Works\r\nConnection: Keep-Alive\r\nContent-Length: #{sz}\r\n\r\n", lines(5)
     assert_equal "Hello", @client.read(5)
   end