Memoize endpoint helpers.

This PR follows the same logic as #1201.

Author: marshall-lee

.rubocop_todo.yml

@@ -22,7 +22,7 @@ Metrics/BlockNesting:
 # Offense count: 5
 # Configuration parameters: CountComments.
 Metrics/ClassLength:
-  Max: 253
+  Max: 257
 
 # Offense count: 23
 Metrics/CyclomaticComplexity:

lib/grape/endpoint.rb

@@ -189,6 +189,7 @@ def compile_path(prepared_path, anchor = true, requirements = {})
 
     def call(env)
       build_app
+      build_helpers
       dup.call!(env)
     end
 
@@ -307,12 +308,17 @@ def build_stack
       b.to_app
     end
 
-    def helpers
-      mod = Module.new
-      (namespace_stackable(:helpers) || []).each do |mod_to_include|
-        mod.send :include, mod_to_include
+    def build_helpers
+      helpers = namespace_stackable(:helpers) || []
+      Module.new do
+        helpers.each do |mod_to_include|
+          include mod_to_include
+        end
       end
-      mod
+    end
+
+    def helpers
+      @helpers ||= build_helpers
     end
 
     def run_filters(filters, type = :other)