Correct captchagid key name for request body.

Noticed a problem there the captcha gid was being incorrectly sent to the steam web server as captcha_gid. The correct key is captchagid.

Attached image of the current steamcommunity.com javascript used for this same auth procedure.
![screen shot 2013-10-03 at 11 34 20 pm](https://f.cloud.github.com/assets/1611098/1267475/0dd090d4-2cbf-11e3-820a-78ca4f332be8.png)