Work with queries that don't have row descriptions

I was trying to upgrade `any-db-postgres` to work with the latest
`pg-query-stream` and discovered that issuing certain queries through a Cursor 
(e.g. `CREATE TABLE` queries) will hang the connection because they do not
cause a `'commandComplete'` or `'rowDescription'` event to be emitted. I'm
still not sure why this doesn't affect the standard `Query` class, but the
change here appears to work for me.

Furthermore, I know that this is a problem of my own design: `any-db-postgres`
issues all textual queries as `QueryStream` instances, but that seemed less
awful than trying to inspect the query to figure out what kind of query to use.