[plugin] director: fixes jail support
[plugin] director: fixes jail support

when passing a req.header('Jail') as second argument to director.pass(),
it will be not a C string but a BufferRef, thus, not being
NULL-terminated. But the length is stored in the number_ member field
of FlowValue already, so let's use it.

xzero/x0,christianparpart/x0,xzero/x0,christianparpart/x0,xzero/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0,xzero/x0,xzero/x0,christianparpart/x0,xzero/x0,christianparpart/x0