(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.