bugfix for #261

Previously, the implementation for spearman's rho chose the highest rank for equal observed values. It should actually take the average rank. As a test, consider:

`(spearmans-rho '(3 3 4 4 4 4 4 4) '(1 1 1 1 2 2 3 4))`

The result should be 0.6607...etc. but under the current implementation, it's returning 0.41666...