Fix client side not to expect 24 bytes connection header

This commit fixes client side code so that it does not expect 24
bytes connection header.

This is because the client connection header is 24 bytes
connection header followed by SETTINGS frame. On the other hand,
the server connection header is SETTINGS frame only.

example/client.rb now sends 24 bytes connection header and also
sends :scheme header field, which is required by the
specification.
client should emit connection header
split connection into separate Server / Client classes