Add SNI support to ESP for https clients

This PR adds the SNI (Server Name Indication) support to ESP for https clients.
Problem solved by this PR: before this PR, ESP https clients do not support SNI for https connections. Details in the following two github issues.
- cloudendpoints#271
- cloudendpoints#262

src/nginx/http.cc

@@ -1002,6 +1002,8 @@ Status initialize_upstream_request(ngx_log_t *log, HTTPRequest *request,
         http_cctx->main_conf[ngx_esp_module.ctx_index]);
     http_connection->upstream_conf.ssl = mc->ssl;
     http_connection->upstream_conf.ssl_session_reuse = 1;
+    // For SNI support
+    http_connection->upstream_conf.ssl_server_name = 1;
     if (mc->cert_path.len > 0) {
       http_connection->upstream_conf.ssl_verify = 1;
     }