Suppress deprecated warning by Object#=~ since ruby 2.6
Suppress deprecated warning by Object#=~ since ruby 2.6

In `Temple::Mixins::GrammerDSL::Value#Rule`, call `=~` method to
`Class` class then that causes a warning message.
This behavior introduced from ruby 2.6 with `-W` option.
(And ruby 2.7 always show a warning message)

- https://bugs.ruby-lang.org/issues/15231
- https://github.com/ruby/ruby/commit/ebff9dc10

Therefore use case-when-else clause to avoid warning.

kbrock/temple,judofyr/temple,k0kubun/temple