Turn numbers into strings when stringifying the timezone data.

When trying to create prebuilt JSON using the IANA files, the africa file was yielding integers for zone data, i.e.  

{"Africa/Algiers":[[-12.2,"-","LMT",-2486678340000], ... ] }

instead of

{"Africa/Algiers":[["-12.2","-","LMT","-2486678340000"], ... ] }

Various functions throughout this library depend on these values being strings.  This PR turns any numbers into strings when stringifying.