Lint rack_builder.rb: avoid naming a method (#1626)

Lint rack_builder.rb: avoid naming a method

The method name gave a complaint

Author: olleolleolle

lib/faraday/rack_builder.rb

@@ -221,7 +221,7 @@ def raise_if_locked
     end
 
     def raise_if_adapter(klass)
-      return unless is_adapter?(klass)
+      return unless klass <= Faraday::Adapter
 
       raise 'Adapter should be set using the `adapter` method, not `use`'
     end
@@ -234,10 +234,6 @@ def adapter_set?
       !@adapter.nil?
     end
 
-    def is_adapter?(klass) # rubocop:disable Naming/PredicateName
-      klass <= Faraday::Adapter
-    end
-
     def use_symbol(mod, key, ...)
       use(mod.lookup_middleware(key), ...)
     end