added NaN test for Phi_approx
added NaN test for Phi_approx

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan