On assigning const to immutable

[Ki Rill]

Why does the first example `class A` work, but the second one 
with `class B` does not?
```D
class A {
     immutable int a;
     this(in int a) {
     	this.a = a;
     }
}

class B {
	immutable int[] b;
     this(in int[] b) {
     	this.b = b;
     }
}

void main()
{
     auto a = new A(1);
     auto b = new B([1, 2]);
}
```

It implicitly converts `const` to `immutable`, but fails to do 
the same with an array. Is it intentional? Why?