Test and fix for schema inheritance bug

When extending one model from another, the old behavior (e.g. rev. 974469c) was to merge the schema of the parent model into that of the child model. The new function _initialize in data/Model.php breaks this behavior (the schema information for the parents is collected, but never merged with the child schema). This pull request contains a test for this behavior and a one-line addition to data/Model.php that restores the old behavior.