HTTP/2: improved handling of END_STREAM in a separate DATA frame.

The save body filter saves the request body to disk once the buffer is full.
Yet in HTTP/2 this might happen even if there is no need to save anything
to disk, notably when content length is known and the END_STREAM flag is
sent in a separate empty DATA frame.  Workaround is to provide additional
byte in the buffer, so saving the request body won't be triggered.

This fixes unexpected request body disk buffering in HTTP/2 observed after
the previous change when content length is known and the END_STREAM flag
is sent in a separate empty DATA frame.

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 461bbff..a037e7a 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -4034,6 +4034,9 @@
 
     if (len < 0 || len > (off_t) clcf->client_body_buffer_size) {
         len = clcf->client_body_buffer_size;
+
+    } else {
+        len++;
     }
 
     if (r->request_body_no_buffering && !stream->in_closed) {