Use toString() instead of toUTCString()

Hi, I'm using `debug` to debug my modules, but GMT is not suitable for me.
It's very nice of you to accept this PR to use the following format:
```
Wed Mar 19 2014 11:25:04 GMT+0800 (CST) mymodule setting request
```
Instead of:
```
Wed, 19 Mar 2014 03:13:20 GMT mymodule setting request
```