[fix #208] fix a bug where set a constant which it will not raise an exception if it contains Fixnum in namaspec

Result:
$  macruby -e 'p 7::C = 42'
42
$ macruby -e 'A = 7; p A::C = 42'
42

Expect:
$ ruby -e 'p 7::C = 42'
-e:1:in `<main>': 7 is not a class/module (TypeError)
$ ruby -e 'A = 7; p A::C = 42'
-e:1:in `<main>': 7 is not a class/module (TypeError)