_text and _html attributes and methods in FooterSettings do not match

[airyxiang]

According to the comments, the _text and _html attributes in FooterSettings are of type str, which will cause an error when calling the get() method of mail.

/usr/local/lib/python3.10/site-packages/sendgrid/helpers/mail/footer_settings.py in get(self)
88
89 if self.text is not None:
---> 90 footer_settings["text"] = self.text.get()
91
92 if self.html is not None:

AttributeError: 'str' object has no attribute 'get'