[Pixiv] Treat absent s_mode as exact match in search

[nisehime]

On pixiv search there are 3 modes: tags partial, tags exact and title+description. On the web version it's specified by s_mode param in the query. Currently though, if you set tags exact in the UI, s_mode will be omitted. In other words if you use URL like https://www.pixiv.net/en/tags/女の子/artworks it will show exact matches results. But gallery-dl treats absent s_mode as partial match, so the results will be different from web version.