Minor tweaks to Open Library subject recommendation

[eocarragain]

two minor tweaks to fix issues we encountered:
-- ensure the json response has info on works
-- strip slashes from subject string

[demiankatz]

I've merged this with a couple of minor tweaks -- an extra isset() check to be sure notices are not triggered, and a simplification of the str_replace code to eliminate a couple of unnecessary lines. Do you mind re-testing against the current master to make sure I didn't break anything?

Thanks for sharing this!

[eocarragain]

Thanks. Looks good