Restore dropped fix for #957

@rfnx fixed #957 in acb89a, but this was accidentally merged to master
instead of trunk-tracking.  I checked for this sort of problem as part
of the 1.10 release, but missed this commit.  Restored.

Fixes #1054

src/ngx_pagespeed.cc

@@ -2479,8 +2479,8 @@ ngx_int_t send_out_headers_and_body(
 
   rc = ngx_http_send_header(r);
 
-  if (rc != NGX_OK) {
-    return NGX_ERROR;
+  if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
+    return rc;
   }
 
   // Send the body.