moved test for nan into its own function
adding failing tests for as_bool with integer argument
updated implementation of as_bool; removed int version, changed return type to bool
updated implementation of as_bool; removed int version, changed return type to bool
Merge branch 'develop' into feature/issue-852-as_bool
Merge pull request #869 from stan-dev/feature/issue-852-as_bool

Fixes #852. Feature/issue 852 as bool