[perl #125791] Ranges can be modified through .bounds in Rakudo; supposed to be immutable

[Christian Bartolomaeus via RT]

I adjusted the test with commit 17d9681 so that it passes on JVM as well. I'm 
closing this ticket as 'resolved'.

[Please note: The change to the test was needed because X::Assignment::RO 
didn't get the typename right on JVM. That has nothing to do with .bounds as 
shown here:

$ perl6-m -e '2 = 3'
Cannot modify an immutable Int
  in block <unit> at -e:1

$ perl6-j -e '2 = 3'
Cannot modify an immutable value
  in block <unit> at -e:1
]