Render the body of the message from a string. If ``rich`` isn’t 
        provided then the message will not have a rich body.

def render_template_string(self, plain, rich = None, **context):
        '''Render the body of the message from a string. If ``rich`` isn’t 
        provided then the message will not have a rich body.'''
        self.plain = render_template_string(plain, **context)
        if rich is not None:
            self.rich = render_template_string(rich, **context)