flexirest: Add Flexirest::BaseWithoutValidation#initialize

`Flexirest::BaseWithoutValidation#initialize` can take `attrs` to build
a response object.

https://github.com/flexirest/flexirest/blob/v.10.6/lib/flexirest/base_without_validation.rb#L22

Author: tk0miya

gems/flexirest/1.12/base_without_validation.rbs

@@ -9,6 +9,7 @@ module Flexirest
     include Caching
     extend Caching::ClassMethods
 
+    def initialize: (?untyped attrs) -> void
     def self._request: (String, Symbol, *untyped, **untyped) -> untyped
     def self._plain_request: (String, Symbol, *untyped, **untyped) -> untyped
     def self.prepare_direct_request: (String, Symbol, **untyped) -> untyped