refactor(MergeMapTo): clarify 'any' types in MergeMapToSubscriber (#1749)
refactor(MergeMapTo): clarify 'any' types in MergeMapToSubscriber (#1749)

* refactor(MergeMapTo): clarify 'any' types in MergeMapToSubscriber

* refactor(MergeMapTo): uniform the argument type with the definitions of the operator

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