small change

Gary:

I made a tiny change in the go-oauth code you wrote.  It is working well for me and I've integrated it as my oauth1 support in my web toolkit.  (Seven5 is the toolkit, hoping to push out a new release in the next couple of weeks.)

Anyway, when integrating go-oauth with evernote I found that evernote doesn't supply the oauth_token_secret at the 2nd token exchange.  This is a documented behavior on their website, so I suppose they know what they are doing...

Please merge if you want, otherwise I'll leave it in my account.

thanks
ian