integrate_ode_bdf tests mostly working now (Issue #1887)

Author: bbbales2

stan/math/prim/fun/value_of.hpp

@@ -107,7 +107,7 @@ inline Vec value_of(Vec&& x) {
 template <typename EigMat, require_eigen_t<EigMat>* = nullptr,
           require_not_vt_double_or_int<EigMat>* = nullptr>
 inline auto value_of(const EigMat& M) {
-  return M.unaryExpr([](const auto& scal) { return value_of(scal); });
+  return M.unaryExpr([](const auto& scal) { return value_of(scal); }).eval();
 }
 
 /**