Move call to validate constructor in ComplexTypeModelBinder into CreateModel

[rynowak]

See commit here: 842d661

This check should be in CanCreateModel so that it will be skipped for someone who subclassed ComplexTypeModelBinder

Also, this change adds runtime reflection to pretty hot codepath just for validation purposes. We can do this one time per-instance by moving it to the inside of this block:

if (_modelCreator == null)	
{
}

[rynowak]

/cc @kichalla @dougbu

[dougbu]

@rynowak you're talking about a bigger refactorization of ComplexTypeModelBinder than immediately evident. For one thing, the _modelCreator == null check is in CreateModel() and not CanCreateModel(); moving the constructor check there would be kinda late. For another, CanCreateModel() has a couple of successful return paths that'll need to change.

Of course, if the uncached reflection shows up in profiles, party on.

[rynowak]

This check should be in CanCreateModel so that it will be skipped for someone who subclassed ComplexTypeModelBinder

I'm suggesting we fix this scenario, which we've directed people to do, as part of the patch.