title: How to resolve "Invalid signature. Expected signature base string" in OAuth 1.0
body: <p>I'm attempting to get an access token and secret from a site using OAuth. The exchange of request token and request secret goes fine, but when it comes time to get the access tokens I get the error <code>"Invalid signature. Expected signature base string."</code></p>
  
  <p>Has anyone seen this error before or know what might be wrong? Here is the data I am getting back (after <code>urldecode</code>-ing it):</p>
  
  <pre><code>Invalid signature. Expected signature base string: POST 
  https://www.readability.com/api/rest/v1/oauth/access_token 
  oauth_consumer_key=my_consumer_key 
  oauth_nonce=d9aff6a0011a633253c5ff9613c6833d79d52cbe 
  oauth_signature_method=HMAC-SHA1 
  oauth_timestamp=1311186899 
  oauth_token=C8GF7D6ytPzQKdZVpy 
  oauth_verifier=ncUV4tJSrS 
  oauth_version=1.0 
  signature=7jUuk6fsEL8XNYxVWcsfGXEreK0%3D 
  </code></pre>