Feature/test nan as bool

#### Summary:
Added NaN tests for as_bool in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN (in code since the function is not exposed to users). 

#### Intended Effect:
Added NaN tests for as_bool in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN (in code since the function is not exposed to users). 

#### How to Verify:
Run test/unit-agrad-fwd/functions/as_bool, test/unit-agrad-rev/functions/as_bool

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of as_bool at NaN in code.

#### Reviewer Suggestions:
Anyone.