Update negative_binomial_2_cdf to use proper metaprogrammed return type instead of raw var
Merge branch 'feature/issue-676-generalize-operandsandpartials' of https://github.com/stan-dev/stan into feature/issue-676-generalize-operandsandpartials

Conflicts:
	src/test/unit-distribution/univariate/discrete/neg_binomial_2/neg_binomial_2_cdf_test.hpp