record union breaks type information

```ruby
# RBS
# type simple = { flags: Integer }

a1 = { flags: 1 } #: simple
a1[:flags] #=> steep will correctly identify this as an integer

# type not_so_simple = simple & { id: String }
a2 = {id: "abc", flags: 2 }#: not_so_simple
a2[:flags] #=> steep will think this is either a String or an Integer. this is the bug!
```