Fix warnings on gcc 4.4

Added a special version of sp_forward for an outdated version of rvalue references supported by gcc 4.4. The compiler would create a temporary and return an rvalue reference to it in the original code. This resulted in warnings about 'returning reference to temporary'.

The added version is similar to std::forward on that compiler, except it doesn't prohibit template argument deduction (which is in line with the original sp_forward).