UnsupportedOperationException in LazyAggregateCombineFn.mergeAccumulators

Another thing that works on the direct runner but fails on Dataflow: LazyAggregateCombineFn.mergeAccumulators [1] throws an UnsupportedOperationException because the "remove" method is not implemented in CombineFns$ProjectionIterable [2]. We will have to modify the implementation of LazyAggregateCombineFn.mergeAccumulators accordingly.

[1] https://github.com/apache/beam/blob/8a622651edb3a2054865ff411579d3e04c02629a/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/LazyAggregateCombineFn.java#L78

[2] https://github.com/apache/beam/blob/8a622651edb3a2054865ff411579d3e04c02629a/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/CombineFns.java#L642