Model.save() resets internal modified state on error

[glebkhol]

Model.save() resets internal modified state even if Model.update() it calls is unsuccessful. This makes it impossible to handle E11001 "duplicate key on insert" by retrying saving a document again.

In the excerpt of Model.save() below, one can see that modified state is reset unconditionally prior to the update operation.

    if (delta) {
      if (delta instanceof Error) return complete(delta);
      var where = this.$__where(delta[0]);
      this.$__reset();
      this.collection.update(where, delta[1], options, complete);
    } else {
      this.$__reset();
      complete(null);
    }