Set discriminator key at construction time instead of after save

[kevindente]

Kind of a minor issue, but there are times when it would be nice if the discriminator key was available immediately after the model was constructed, rather than waiting until after a Save has been performed.

[talha-asad]

+1

[kevinswarner]

+1 This might solve an issue that I have when constructing documents and working with them before they actually get saved.

[sieira]

I will check it, but it may be hard to fix, it will take some time.