Remove direct struct subclassing (#1491)

This is a follow-up to 39f1b35 - I noticed I was still missing Faraday::Request#body= from Tapioca's generated definitions. Also did Faraday::Adapter::Test::Stub while we're here for consistency.

lib/faraday/adapter/test.rb

@@ -184,7 +184,7 @@ def matches?(stack, env)
       end
 
       # Stub request
-      class Stub < Struct.new(:host, :path, :query, :headers, :body, :strict_mode, :block) # rubocop:disable Style/StructInheritance
+      Stub = Struct.new(:host, :path, :query, :headers, :body, :strict_mode, :block) do
         # @param env [Faraday::Env]
         def matches?(env)
           request_host = env[:url].host

lib/faraday/request.rb

@@ -24,13 +24,14 @@ module Faraday
   #   @return [String] body
   # @!attribute options
   #   @return [RequestOptions] options
-  #
-  # rubocop:disable Style/StructInheritance
-  class Request < Struct.new(:http_method, :path, :params, :headers, :body, :options)
-    # rubocop:enable Style/StructInheritance
-
+  Request = Struct.new(:http_method, :path, :params, :headers, :body, :options) do
     extend MiddlewareRegistry
 
+    alias_method :member_get, :[]
+    private :member_get
+    alias_method :member_set, :[]=
+    private :member_set
+
     # @param request_method [String]
     # @yield [request] for block customization, if block given
     # @yieldparam request [Request]
@@ -48,7 +49,7 @@ def params=(hash)
       if params
         params.replace hash
       else
-        super
+        member_set(:params, hash)
       end
     end
 
@@ -59,7 +60,7 @@ def headers=(hash)
       if headers
         headers.replace hash
       else
-        super
+        member_set(:headers, hash)
       end
     end