(maint) Fix spec for UTF-8 strings in user comment properties

The first problem with this spec is that it wasn't actually
being run due to an always false "if" clause. It was checking
for String responding to encode, but encode is an instance
method. method_defined? should have been used instead.

The second problem with this spec is that the test value did not
contain any non-ASCII characters. If it had, it would have caught
the bug described in PUP-1473. Added a non-ASCII character to test
for regressions.