JSON.parse fails on db.head() when document is missing

[slaskis]

Maybe it's only when I was mucking about with a design document, but the HEAD-request didn't give an error so it failed on this line trying to JSON.parse undefined.

I 'fixed it' by changing that particular line to this:

        rev: JSON.parse(headers.etag||'""'),

I should probably fork and give you pull requests. But this was so tiny, so...next time! :)