Changed model_id method to look up the model_id method using the class name (string) rather than the class itself.
Changed model_id method to look up the model_id method using the class name
(string) rather than the class itself.

This fixes a problem that sometimes occurs in the development environment when
a class gets reloaded by Rails. When that happens, the class object referenced
by the constant (User, for example) is a *different* object than the class
object referenced by IndexedSearch::Index.models_by_id, so
IndexedSearch::Index.models_by_id.invert[self] returns nil, model_id returns
nil, and this line in make_search_insertions fails:
     idx = ((id << 8) | model_id)

dburry/indexed_search,dburry/indexed_search