Change the header parse function not to exclude ending `-n` character in string

in po.js on line 84 you parse the value of a header.

you do this via:

```
value = p.join(':').trim().replace(/n$/);
```

I wonder why you replace the final character if it is `n` without even supplying no replace character. 

my: `"Language: en\n"` therefor ends up being  Language: eundefined

what is the reason for replacing the final character if it is n, with nothing?
