nghttpx: Fix bug that causes connection failure with backend proxy URI

This is a regression when we introduced SSL/TLS session resumption in
HTTP/2 backend.  Before the introduction of session resumption,
conn_.tls.ssl is always nullptr when connection is made to proxy.  But
we have to keep conn_.tls.ssl to enable session resumption, so our
code breaks when it is reused.  This commit fixes this issue.

See GH-421

src/shrpx_http2_session.cc

@@ -1569,17 +1569,17 @@ int Http2Session::connected() {
 
   conn_.rlimit.startw();
 
-  if (conn_.tls.ssl) {
-    read_ = &Http2Session::tls_handshake;
-    write_ = &Http2Session::tls_handshake;
+  read_ = &Http2Session::read_clear;
+  write_ = &Http2Session::write_clear;
 
+  if (state_ == PROXY_CONNECTING) {
     return do_write();
   }
 
-  read_ = &Http2Session::read_clear;
-  write_ = &Http2Session::write_clear;
+  if (conn_.tls.ssl) {
+    read_ = &Http2Session::tls_handshake;
+    write_ = &Http2Session::tls_handshake;
 
-  if (state_ == PROXY_CONNECTING) {
     return do_write();
   }