HTTP/2: fixed invalid headers handling (ticket #831).

The r->invalid_header flag wasn't reset once an invalid header appeared in a
request, resulting in all subsequent headers in the request were also marked
as invalid.
diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 564f248..e77c13a 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -2949,6 +2949,8 @@
         return NGX_ERROR;
     }
 
+    r->invalid_header = 0;
+
     cscf = ngx_http_get_module_srv_conf(r, ngx_http_core_module);
 
     for (i = (header->name.data[0] == ':'); i != header->name.len; i++) {