Add guard as same as ==

Try to fix failure at https://github.com/ruby/ruby/runs/207580232#step:10:382
```
       undefined method `name' for "Gemfile.lock":String
       /home/runner/work/ruby/ruby/lib/bundler/resolver/spec_group.rb:65:in `eql?'
```

lib/bundler/resolver/spec_group.rb

@@ -62,6 +62,7 @@ module Bundler
       end
 
       def eql?(other)
+        return unless other.is_a?(SpecGroup)
         name.eql?(other.name) &&
           version.eql?(other.version) &&
           source.eql?(other.source)