Filter passes expected columns to __update__
Filter passes expected columns to __update__

Related to the previous commit, FilterResult._step inits an object
then updates fields from the DynamoDB response.  Unlike a table load,
however, queries/scans can specify which attributes should be loaded.

The actual loaded attrs may not match the projection/specific columns
since LSIs will fetch the entire row if the requested attrs are
a superset of the projected attributes.

Ref #4

numberoverzero/bloop,numberoverzero/bloop