bugfix: ngx.re.gsub() mistakely eliminate the first character.

ngx.re.gsub() will mistakely eliminate the first character sometimes.

To reproduce:
```
newstr, n, err = ngx.re.gsub('TestCase', '^ *', '', '')                                                                                                                                     
ngx.say(newstr)
```

Result: 
```
estCase
```