Bugfix/issue 887 fdim rev nan

#### Summary:
Fixes issue #887 . 

#### Intended Effect:
agrad/rev/fdim should now return NaN when either argument is NaN.

#### How to Verify:
Run test/unit-agrad-rev/functions/fdim.

#### Side Effects:
None.

#### Documentation:
None. Should document behavior of function when given NaN together with other functions.

#### Reviewer Suggestions:
Anyone