Patch for a ActiveSupport::JSON.decode bug

Hi all,

I recently ran into the issue reported in ticket 3479.
ActiveSupport::JSON.decode doesn't handle newlines correctly when
using YAML backend.

I attached a patch (with tests) in a comment :

This bug affects every version of activesupport from 2.3.5 (and
probably older too). So the patch could be useful for next "2-3-
stable" release.