Update AuthorizeRequest.php

I've come across an issue with the SendEmail option in 
Omnipay\SagePay\Message\Form\AuthorizeRequest

At https://github.com/thephpleague/omnipay-sagepay/blob/master/src/Message/Form/AuthorizeRequest.php#L127
the SendEmail data is set up, but it's then filtered out because of the mis-match in case sensitivity
at https://github.com/thephpleague/omnipay-sagepay/blob/master/src/Message/Form/AuthorizeRequest.php#L29

So $data['SendEmail'] is set up properly but then filtered out because it's not in the $validFields array
(the $validFields array contains SendEMail but not SendEmail).

I'd propose updating $validFields to allow 'SendEmail' rather than 'SendEMail', but I wanted to check this
with the creators of this useful project :)

Author: davesmall9xb

src/Message/Form/AuthorizeRequest.php

@@ -26,7 +26,7 @@ class AuthorizeRequest extends DirectAuthorizeRequest
         'CustomerName' => false,
         'CustomerEMail' => false,
         'VendorEMail' => false,
-        'SendEMail' => false,
+        'SendEmail' => false,
         'EmailMessage' => false,
         'BillingSurname' => true,
         'BillingFirstnames' => true,