fix error introduced in r1273 and r1275:

nginx did not process FastCGI response if header was at end of FastCGI record

src/http/modules/ngx_http_fastcgi_module.c

@@ -1232,7 +1232,7 @@ ngx_http_fastcgi_process_header(ngx_http_request_t *r)
         }
 
         if (rc == NGX_OK) {
-            return NGX_AGAIN;
+            continue;
         }
 
         /* rc == NGX_AGAIN */