Move #app from Connection to Builder

lib/faraday/builder.rb

@@ -73,6 +73,17 @@ module Faraday
       self.class.new(@handlers.dup)
     end
 
+    def app
+      @app ||= begin
+        lock!
+        to_app(lambda { |env|
+          response = Response.new
+          response.finish(env) unless env.parallel?
+          env.response = response
+        })
+      end
+    end
+
     def to_app(inner_app)
       # last added handler is the deepest and thus closest to the inner app
       @handlers.reverse.inject(inner_app) { |app, handler| handler.build(app) }

lib/faraday/connection.rb

@@ -100,26 +100,7 @@ module Faraday
 
     extend Forwardable
 
-    def_delegators :builder, :build, :use, :request, :response, :adapter
+    def_delegators :builder, :build, :use, :request, :response, :adapter, :app
-
-    # The "rack app" wrapped in middleware. All requests are sent here.
-    #
-    # The builder is responsible for creating the app object. After this,
-    # the builder gets locked to ensure no further modifications are made
-    # to the middleware stack.
-    #
-    # Returns an object that responds to `call` and returns a Response.
-    def app
-      @app ||= begin
-        builder.lock!
-        builder.to_app(lambda { |env|
-          # the inner app that creates and returns the Response object
-          response = Response.new
-          response.finish(env) unless env.parallel?
-          env.response = response
-        })
-      end
-    end
 
     # Public: Makes an HTTP request without a body.
     #