delete does not handle keyPath

[yoann-antoviaque]

The delete function does not check if a keypath is specified, therefore tries to remove the entry using the id attribute making the state incoherent: the object is correctly removed from the backbone collection but not from indexeddb

changing

var deleteRequest = store.delete(json.id);

to

var deleteRequest;
if (!store.keyPath)
      deleteRequest = store.delete(json.id);
else
      deleteRequest = store.delete(object.get(store.keyPath)); 

fix this issue

[julien51]

Please, submit a pull requests with passing tests :)