`path_beg` and `replace_beg` unexpectedly apply to `grip-link` corrupting it

[eduardbareev]

routes file:

*,proto=http,ssl=no,path_beg=/sse,replace_beg=/fooo/baaz/quux 172.18.0.5:80

When client calls pushpin and requests /sse, pushpin requests /fooo/baaz/quux on 172.18.0.5:80,
thus performing expected substitution.

172.18.0.5:80/fooo/baaz/quux then responds with grip-link header set to </s.php>[…].

172.18.0.5:80/fooo/baaz/quux:

[…]
grip-hold: stream
grip-channel: channel_123
grip-channel: channel_777
grip-link: </s.php>; rel=next; timeout=3
[…]

But instead of requesting /s.php, pushpin requests /fooo/baaz/quuxhp.

GET /fooo/baaz/quuxhp HTTP/1.1" 404 8512 "-" "-"

Pushpin was built from pushpin-1.41.0.tar.bz2 tarball.