Invoke flush before close_write in write_body_and_close.

It does not appear guaranteed that `close_write` will flush the underlying
IO buffers, so let's ensure it's flushed.

lib/protocol/http1/connection.rb

@@ -379,6 +379,7 @@ def write_body_and_close(body, head)
 					end
 				end
 
+				@stream.flush
 				@stream.close_write
 			end