http: keep F_LENIENT between messages

`F_LENIENT` flag should not be reset along with the other flags when
starting parsing a new message. This setting should remain on for the
lifetime of the parser or until `llhttp_set_lenient(..., 0)`.

PR-URL: #34
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

src/native/http.c

@@ -74,9 +74,11 @@ int llhttp__after_message_complete(llhttp_t* parser, const char* p,
   int should_keep_alive;
 
   should_keep_alive = llhttp_should_keep_alive(parser);
-  parser->flags = 0;
   parser->finish = HTTP_FINISH_SAFE;
 
+  /* Keep `F_LENIENT` flag between messages, but reset every other flag */
+  parser->flags &= F_LENIENT;
+
   /* NOTE: this is ignored in loose parsing mode */
   return should_keep_alive;
 }

test/request/lenient.md

@@ -22,6 +22,35 @@ off=33 headers complete method=1 v=1/1 flags=100 content_length=0
 off=33 message complete
 ```
 
+## Second request header value with lenient
+
+<!-- meta={"type": "request-lenient"} -->
+```http
+GET /url HTTP/1.1
+Header1: Okay
+
+
+GET /url HTTP/1.1
+Header1: \f
+
+
+```
+
+```log
+off=0 message begin
+off=4 len=4 span[url]="/url"
+off=19 len=7 span[header_field]="Header1"
+off=28 len=4 span[header_value]="Okay"
+off=36 headers complete method=1 v=1/1 flags=100 content_length=0
+off=36 message complete
+off=38 message begin
+off=42 len=4 span[url]="/url"
+off=57 len=7 span[header_field]="Header1"
+off=66 len=1 span[header_value]="\f"
+off=71 headers complete method=1 v=1/1 flags=100 content_length=0
+off=71 message complete
+```
+
 ## Header value without lenient
 
 <!-- meta={"type": "request"} -->