The previous solution removed numbers entirely, whereas this one treats
them like letters and checks if they are evenly spaced.
More importantly, the old solution test seemed to check if the numbers
were palindromic, but because the solution replaced them with "", it
wasn't testing what it seemed to.
I added a new test to differentiate between the palindromic "rac3e3car"
and the non-palindromic "r3ace3car".
These changes also obviate the problems raised in Issue #355.
