Loosen unprintable character sanitization

We were too aggressive with our fixing of #295. [:print:] is catching \r\n, so let's explicitly fix the weird JSON unicode chars (this aligns with the rack fix as well: https://github.com/rack/rack-contrib/pull/37/files).
Merge pull request #314 from swanson/fix-old-mac-line-endings

Loosen unprintable character sanitization to not remove valid line-endings
Add migration to correct unicode issues that were resolved by #295