fix bug: response large(> 4294967295) file fail

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

src/connection.h

@@ -74,7 +74,7 @@ struct uh_connection_internal {
     uint8_t flags;
     struct {
         int fd;
-        int size;
+        size_t size;
     } file;
     struct ev_io ior;
     struct ev_io iow;