Fixed behavior of within to remove special case.

Actually, that doesn't work. The check in the loop is if v < s[i], which will fail if anyway. Consider
s := []float64{2, 2}

The value 2 should return -1. 2 is <= s[0], but it is not < s[1]. I don't think anything can be done. What if 

s := []float64{1,1,1,1}

What should Within(s, 1) return? It seems to me the only reasonable answer is -1.