fixed agrad/rev/round to return gradient NaN for input NaN
fixed agrad/rev/round to return gradient NaN for input NaN

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