convert nan and inf into JSON "null" or CSV '' #13

NaN and +-Inf numbers are not allowed in JSON.  
They should be represented as "null" in JSON output.  
In CSV output, it's less clear since there is no standard, but empty string 
would work for me.

see http://tools.ietf.org/html/rfc4627 about NaN

see http://en.wikipedia.org/wiki/IEEE_754-2008 about JSON numbers
2.4.  Numbers
...
Numeric values that cannot be represented as sequences of digits
   (such as Infinity and NaN) are not permitted.