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?
You technically don't need the `Math.max` here, since you have the assertion above that ensures that `len1` is equal to `len2`. You could just use `len1` or whatever. That being said, I think @kytrinyx said in the past that the example solutions don't need to be ideal solutions, just anything that passes the tests, so yeah, no biggie either way.