Return the buffer contents of the frame.

def return_buffer_contents(self, frame, force_unescaped=False):
        """Return the buffer contents of the frame."""
        if not force_unescaped:
            if frame.eval_ctx.volatile:
                self.writeline('if context.eval_ctx.autoescape:')
                self.indent()
                self.writeline('return Markup(concat(%s))' % frame.buffer)
                self.outdent()
                self.writeline('else:')
                self.indent()
                self.writeline('return concat(%s)' % frame.buffer)
                self.outdent()
                return
            elif frame.eval_ctx.autoescape:
                self.writeline('return Markup(concat(%s))' % frame.buffer)
                return
        self.writeline('return concat(%s)' % frame.buffer)