int64 over 0xffffffff (negative) are not stringified when encoded to JSON

A json-serialized `uint64` gets swapped for a string over 0xffffffff, but not when a int64 has a larger than 32 bits value, but is negative.

That's in https://github.com/EOSIO/fc/blob/master/src/io/json.cpp#L614

Example:

```
"int64": -2714170076832633519,
"uint64": "2714170076832633519",
```