fixed crash on saving (iOS4)

When `NSJSONSerialization` is not available (iOS4), HTTP body will be assembled by `pkt_URLEncodedFormString:`. The argument (`NSDictionary`) may be contain `NSNumber`, but this method does not transfer the type of it. This leads to crash in `[PocketAPIOperation encodeForURL:value]`.

I avoided this issue by transferring the type.


#### example of argument
```
key(NSString): time
value(NSNumber): 1361867614

key(NSString): consumer_key
value(NSString): *****

key(NSString): url
value(NSString): http://www.example.com

key(NSString): access_token
value(NSString): *****
```