fix exception when chrome-cli executes js to extract json object or numb...

this commit fixes exeptions when chrome-cli executes js to extract json object or number.

```
$ chrome-cli execute '1'
2014-02-14 21:34:27.129 chrome-cli[35875:707] -[__NSCFNumber UTF8String]: unrecognized selector sent to instance 0x187
2014-02-14 21:34:27.130 chrome-cli[35875:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber UTF8String]: unrecognized selector sent to instance 0x187'
...
```