Unordered: Fix unnecessary_copy_tests for MSVC10
Compilers with rvalue references can avoid creating a node, as they can use the
value from the rvalue reference to check if the value is already in the
container (in this case it is) before creating the node. Could possibly do the
same for compilers without rvalue references, if it can get a value out of
Boost.Move's rvalue reference emulation.