BigInteger parsing is inconsistent with Long parsing

```
String json = "1000.0";

new Gson().fromJson(json, long.class); // 1000L
new Gson().fromJson(json, BigInteger.class); // throws

// expected BigInteger.valueOf(1000L) instead

What version of the product are you using? On what operating system?

2.2.4
```

Original issue reported on code.google.com by `isaac.q....@gmail.com` on 21 Apr 2014 at 3:35