Fix add_header's handling of dicts

sendgrid/helpers/mail/mail.py

@@ -464,7 +464,7 @@ def add_header(self, header):
         :param value: A Header object or a dict of header key/values
         :type value: Header, dict
         """
-        if header.personalization is not None:
+        if isinstance(header, Header) and header.personalization is not None:
             try:
                 personalization = \
                     self._personalizations[header.personalization]