validateBSON should assert that Array elements have indexes from 0...n

According to the BSON spec, "The document for an array is a normal BSON document with integer values for the keys, starting with 0 and continuing sequentially". Currently, we validate BSON in validateBSON. validateBSON treats arrays exactly like nested BSON. We should add an additional check, which verifies that the first index is '0', and each subsequent key is sequential.