Hopefully fix CI errors where constant not found

lib/active_model/serializer/adapter.rb

@@ -30,9 +30,6 @@ def get(adapter)
           # 2. return if registered
           adapter_map.fetch(adapter_name) {
             # 3. try to find adapter class from environment
-            # We want to avoid the adapter.to_s.classify since it depends on the
-            # Inflector. For this reason, we cannot use get_const nor
-            # safe_constantize.
             adapter_class = find_by_name(adapter_name)
             if adapter_class
               register(adapter_name, adapter_class)
@@ -44,11 +41,13 @@ def get(adapter)
           }
         end
 
+        # We want to avoid the adapter.to_s.classify since it depends on the
+        # Inflector, and may incorrectly inflect api to API, rather than Api.
         def find_by_name(adapter_name)
           adapter_const = self.constants.find {|adapter_klass|
             adapter_klass.to_s.downcase.underscore === adapter_name
           }
-          adapter_const && self.const_get(adapter_const.to_sym)
+          adapter_const && "ActiveModel::Serializer::Adapter::#{adapter_const}".safe_constantize
         end
         private :find_by_name