Fix UserFileDefaultOptions#fetch implementation

The usage of `||` fetch is incorrect,
as presence of `false` is the correct value.

This change uses `default` value,
only if the value is nowhere else defined.

lib/puma/configuration.rb

@@ -54,17 +54,18 @@ def initialize(user_options, default_options)
     attr_reader :user_options, :file_options, :default_options
 
     def [](key)
-      return user_options[key]    if user_options.key?(key)
-      return file_options[key]    if file_options.key?(key)
-      return default_options[key] if default_options.key?(key)
+      fetch(key)
     end
 
     def []=(key, value)
       user_options[key] = value
     end
 
     def fetch(key, default_value = nil)
-      self[key] || default_value
+      return user_options[key]    if user_options.key?(key)
+      return file_options[key]    if file_options.key?(key)
+      return default_options[key] if default_options.key?(key)
+      nil
     end
 
     def all_of(key)