Fixed casting of discriminatorKey defined Models

[flore2003]

Fixes casting of Model object types by discriminatorKey

[voz]

@briankircho is there anything preventing this from being merged?

[chopachom]

This breaks the code that uses models that aren't extended with mongoose-schema-extend:

Fatal error: Schema hasn't been registered for model "undefined".
Use mongoose.model(name, schema)

[voz]

I do confirm the issue:

MissingSchemaError: Schema hasn't been registered for model "undefined".
Use mongoose.model(name, schema)

Maybe @flore2003 could have a look

[flore2003]

This is due to #17, as every schema now contains the discriminatorKey '__t' field. This should either be fixed in mongoose (not likely) or mongoose-schema-extend should use another field name (something like just discriminator). #19 fixes this of course, because it catches the discriminatorKey being specified but missing from the data. I don't know if it's the right way to do it, though.