rework plus() for perfect forwarding

Author: rok-cesnovar

stan/math/prim/fun/plus.hpp

@@ -14,8 +14,8 @@ namespace math {
  * @return result of unary plus of the input.
  */
 template <typename T>
-inline plain_type_t<T> plus(const T& x) {
-  return x;
+inline T plus(T&& x) {
+  return std::forward(x);
 }
 
 }  // namespace math