We have extended the general polyatomic canonical variational theory formalism of Isaacson and one of the authors to improved canonical and microcanonical variational theory. We have calculated the rate constants for the reaction in the title over the temperature range 200-2500 K using all three variational theories and the Melius-Blint ab initio potential energy surface. The results are compared to canonical variational calculations based on the reaction-path interpolation scheme of Quack and Troe, to the trajectory calculations of Miller, and to experiment. We find that the microcanonical variational transition states have a strong energy dependence and the generalized free energy of activation curves have two maxima. Quantization effects appear to be important at the lower temperatures, and recrossing effects may be important at higher temperatures.