spam? should send any HTTP_ parameters except COOKIE to akismet

Please do a little code review, but this will send along any HTTP_* headers (except COOKIE) to Akismet. In my support tickets with them they said these additional data points are extremely useful.

```
> So just to be clear I can pass in any CGI header as a parameter? As in:
>
> &HTTP_ACCEPT=text%2Fhtml%2Capplication%2Fxhtml%2Bxml%2Capplication%2Fxml%3Bq%3D0.9%2C*%2F*%3Bq%3D0.8
>
> If so, do you have a complete list of HTTP_ headers you use? Sorry, the
> HTTP protocol is far from my programming strongpoint.

Yes, you can send any header like that.  There's no fixed list, you can just send anything starting with HTTP_ - some spambots and proxies add arbitrary headers, so that ensures we can see everything.

Oh, you should filter out HTTP_COOKIE and anything related to authentication - obviously there's a security risk in sending that to us.
```