title: invalid signature when attempting a sell
body: <p>I'm try to sell my bitcoin through the API from a specific wallet.</p>
  
  <p>I have the GET calls working correctly, but the POST call is showing "invalid signature"</p>
  
  <pre><code>$API_KEY        = '*****';
  $API_SECRET = '*****';
  
  $data['amount'] = 10;
  $data['currency']   = 'USD';
  $data['quote']      = true;
  
  $api_url    = '/v2/accounts/593b0285-****-****-****-f0ddcb6c9ccc/sells';
  $timestamp  = time();
  $signature  = hash_hmac('SHA256',$timestamp.'POST'.$api_url.json_encode($data),$API_SECRET;
  $version    = '2017-11-11';
  $headers    = array('CB-ACCESS-SIGN: '.$signature,
                     'CB-ACCESS-TIMESTAMP: '.$timestamp,
                     'CB-ACCESS-KEY: '.$API_KEY,
                     'CB-VERSION: '.$version);
  
  $ch     = curl_init(); 
  curl_setopt($ch, CURLOPT_URL, 'https://api.coinbase.com'.$api_url);
  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
  curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($ch, CURLOPT_POST, true);
  curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
  $data   = curl_exec($ch);
  </code></pre>
  
  <p>This is the result I get from Coinbase</p>
  
  <pre><code>stdClass Object
  (
      [errors] =&gt; Array
          (
              [0] =&gt; stdClass Object
                  (
                      [id] =&gt; authentication_error
                      [message] =&gt; invalid signature
                  )
          )
  )
  </code></pre>
  
  <p>I've changed the currency to BTC to try that.
  The account id in the $api_url is the one from when I perform an account API call.</p>
  
  <pre><code>[id] =&gt; 593b0285-****-****-****-f0ddcb6c9ccc
  [name] =&gt; BTC Wallet
  [primary] =&gt; 1
  [type] =&gt; wallet
  </code></pre>