HTTP/2: invalid connection preface logging (ticket #1981).

Previously, invalid connection preface errors were only logged at debug
level, providing no visible feedback, in particular, when a plain text
HTTP/2 listening socket is erroneously used for HTTP/1.x connections.
Now these are explicitly logged at the info level, much like other
client-related errors.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 8b0fc53..08d66c9 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -731,9 +731,8 @@
     }
 
     if (ngx_memcmp(pos, preface, sizeof(preface) - 1) != 0) {
-        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, h2c->connection->log, 0,
-                       "invalid http2 connection preface \"%*s\"",
-                       sizeof(preface) - 1, pos);
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "invalid connection preface");
 
         return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_PROTOCOL_ERROR);
     }
@@ -754,9 +753,8 @@
     }
 
     if (ngx_memcmp(pos, preface, sizeof(preface) - 1) != 0) {
-        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, h2c->connection->log, 0,
-                       "invalid http2 connection preface \"%*s\"",
-                       sizeof(preface) - 1, pos);
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "invalid connection preface");
 
         return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_PROTOCOL_ERROR);
     }