fix(kernel): Correctly return instances of un-exported types

When an un-exported type that extends an exported type is returned with
an interface as the declared type, the JSII kernel used to return a ref
with the FQN of the exported supertype, instead of correctly wrapping
the instance in a proxy of the interface type as it should have.

Fixes #302

packages/jsii-kernel/lib/kernel.ts

@@ -961,7 +961,9 @@ export class Kernel {
             // have an object id, so we need to allocate one for it.
             this._debug('creating objref for', v);
             const fqn = this._fqnForObject(v);
-            return this._createObjref(v, fqn);
+            if (!targetType || !spec.isNamedTypeReference(targetType) || fqn === targetType.fqn) {
+                return this._createObjref(v, fqn);
+            }
         }
 
         // if the method/property returns an object literal and the return type