I fixed a bug in existing code (see the patterns for get, set, and delete above this).

The purpose is to query a kvs as to whether the model data has a given field. What I don't know is if the semantics are "is this a legitimate field" or "does this field have a current value". The code seems to imply the latter which means that rather than our code doing field in xblock._model_data we should perhaps be calling xblock.has(field) but I'm not sure that would delegate down to the kvs's has method.