http2: Fix missing nghttp2_session_send call in Curl_http2_switched

Previously in Curl_http2_switched, we called nghttp2_session_mem_recv to
parse incoming data which were already received while curl was handling
upgrade.  But we didn't call nghttp2_session_send, and it led to make
curl not send any response to the received frames.  Most likely, we
received SETTINGS from server at this point, so we missed opportunity to
send SETTINGS + ACK.  This commit adds missing nghttp2_session_send call
in Curl_http2_switched to fix this issue.

Bug: https://github.com/bagder/curl/issues/192
Reported-by: Stefan Eissing