Added n = 0, n = 1 tests to negative binomial pdf (Issue #1763)
Added n = 0, n = 1 tests to negative binomial pdf (Issue #1763)

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