Hash each item before doing the comparison.

By taking a hash of the two strings we can guarantee that they'll be the same length when compared by the constant time function. This is a better timing attack mitigation as it removes the branch which has a different instruction count ([see here](https://github.com/codegangsta/martini-contrib/commit/abc2f6607a7f8526b0dea8e1eccf428544f08e49#diff-2144f4c07d50503617dfb64893eaf7b1R13)).