Spans can't be searched filtering with tagQuery and spanName when using cassandra as storage

[qtopie]

Describe the Bug

As titled.

Steps to Reproduce

1. Invoke API of apps to generate some trace data to report.
2. Search with tagQuery such as tagQuery=http.method=GET, spans related will appear in results.
3. Search with spanName such as spanName=get /zipkin/api/v2/traces, spans related will appear in results.
4. Search with tagQuery and spanName, even if there exists data after filtered by step 2,3, but search result is empty.

Expected Behaviour

There should be matched results of span data.