HTTP/2: prevented creating temp files for requests without body.

The problem was introduced by 52bd8cc17f34.

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 53a9c26..421ec6e 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -3537,8 +3537,10 @@
         rb->buf = ngx_create_temp_buf(r->pool, (size_t) len);
 
     } else {
-        /* enforce writing body to file */
-        r->request_body_in_file_only = 1;
+        if (stream->preread) {
+            /* enforce writing preread buffer to file */
+            r->request_body_in_file_only = 1;
+        }
 
         rb->buf = ngx_calloc_buf(r->pool);