title: Braintree sandbox transaction.sale gives error: "Sale Customer ID has already been taken"
body: <p>I am trying out Braintree in sandbox mode using the Drop-in feature. 
  I am creating a Client token using an existing <code>customerID</code>. But when I make <code>transaction.sale</code> call with this <code>customerId</code> and the option <code>storeInVault = true</code> it gives error the following error:</p>
  
  <blockquote>
    <p>Sale Customer ID has already been taken.</p>
  </blockquote>
  
  <p>As per the documentation it should update the customer with the payment nonce.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>gateway.transaction.sale({
    amount: '10.00',
    paymentMethodNonce: nonceFromTheClient, // Generated nonce passed from client
    customer: {
      id: 232057823,  //this customer exist in the vault
      email : user.emails[0].address
    },
    options: {
      submitForSettlement: true,
      storeInVault: true 
      //storeInVaultOnSuccess: true 
    }
  }, function (err, result) {
    if (err) {
      console.log(err);
    } else {
      if (result.success) {
           return result.success;
      } else {
              console.log('ERR Sale '+result.message);
              return result.success;
      }
    }
  });
  </code></pre>
  
  <p>I'm using Meteor with the package <code>patrickml:braintree</code>.</p>