add test case for adding an array of two pairs to a sorted set
Fix incorrect check in zadd array arguments

We were presuming if the arguments array had two elements then it was [2, "value"], but [[2, "value"], [3, "value2"]] are also valid arguments. We now check that the first element isn't an array if there's two elements in the arguments array. Fixes the spec in the previous commit.
Merge branch 'bugfix-array-arguments-for-zadd'