SendGrid: Set to field when using merge_data (#14)

* Set to field when using merge_data

The `to` field is required even if providing recipient addresses in x-smtpapi. See https://sendgrid.com/docs/API_Reference/Web_API/mail.html#-send.

* Check data['to'] contains expected emails

* Add space for toname check

* Make `to` expected data contain email only

anymail/backends/sendgrid.py

@@ -100,6 +100,7 @@ class SendGridPayload(RequestsPayload):
             self.set_recipients('to', self.to_list)
         else:
             # Merge-friendly smtpapi 'to' field
+            self.set_recipients('to', self.to_list)
             self.smtpapi['to'] = [email.address for email in self.to_list]
             self.all_recipients += self.to_list