fixup some type something about DataObjectHandle payload
before this commit a MergingTransformer could only retrieve a Gaudi::Functional::details::vector_of_const_<NON_CONST_TYPE> and would thus operate with const pointers to non-const objects on the TES. Code copy and pasted from lhcb/LHCb!1914 (comment 2580579).
prerequisite for lhcb/LHCb!1914 (closed)