correctly compute hamming for strands of unequal length

Since the hamming distance is equal to the difference between two homologous DNA strands I wanted to add a test that checked the solution was properly limiting the upper bound of the point comparisons.

This cropped up in a nitpick for code like this

`var len = Math.min(strand1.length, strand2.length);`

which suggested the use of `Math.max()` over `Math.min()`. With the present test suite,`Math.max()` makes all the tests pass, but it inflates the hamming distance because it includes point comparisons that should not be included.

Any thoughts?
That line was bugging me too! 

I ended up going with `Math.max` because in my mind it was better to have `len1` and `len2` together rather than having a lone wolf variable. 

Using one variable to make the assignment is probably more efficient, but I didn't want it to seem like there was anything special about either `len1` or `len2` on their own, which is kind of what got me started on this pr in the first place.