String Keys Break Calls

Behold:

``` ruby
[29] pry(main)> @api.accounts.create('email' => 'x', 'password' => 'x')
ArgumentError: comparison of Array with Array failed
from /home/ben/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/oauth-0.4.7/lib/oauth/helper.rb:37:in `sort'
```

If they keys are `Symbol`s, the OAuth gem doesn't blow up. Need to investigate why and make it not so. Might involve converting all hash keys to symbols before sending them to dependency libraries. Haven't confirmed yet, either, whether other calls are affected.