Fix bug with false values

Dan,

I just noticed that there is a bug that occurs when the master has a `false` value for an attribute while the slave has a `true` value.

This is a quick fix to illustrate the problem. The new "does not overwrite false values in master" spec is technically redundant (updating the "uses #assign_attributes to update master" spec catches the problem), but I feel it's a bit clearer IMHO.

Best wishes,
Ben