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?
> Either of those would make the reader wonder why you were just using len1 and ignoring len2.

Yes, I agree with this line of thinking. This LGTM :+1: