is_sampled is always False

[AlllenShen]

The is_sampled logic there will make it always be False.
If the view function is the first span in a trace, X-B3-Sampled is not definded in headers so it is set as Fasle. And then all spans in the call chain will be False.

I use it in two defferent flask applications and no one send trace info to server. When I changed this line to is_sampled = str(headers.get('X-B3-Sampled') or '1') == '1', it worked.