Value of type boxed more than once is not correctly serialized

[dahlia]

Where:

boxed a (text);
boxed b (a);
boxed c (b);

The following Python code depending on the generated Python code doesn’t work as we expect:

assert C(B(A('test')).__nirum_serialize__()) == 'test'
assert B(A('test')).__nirum_serialize__() == 'test'

Although the following code works as we expect:

assert A('test').__nirum_serialize__() == 'test'