pandas.json handling of integers in floating point arrays
cc @Komnomnomnom 

I noticed a discrepancy between `JSON.stringify` in Chrome and both the built-in `json` package (python 2.7 at least) and `ultrajson`. In short the value `4.0`, say, encodes as `"4"` in Chrome and `"4.0"` in the others. In JavaScript-land, this has no practical impact, but I thought I'd bring it up in case others felt we should tweak ultrajson to conform. 
