Don't cache Class.instance_methods

Fix: #2258

Some gems or app code may define methods on `Class` after `grape` is
loaaded but before `override_all_methods!` is called.

As such it's better to check `Class.method_defined?` when doing the override.

lib/grape/api.rb

@@ -8,7 +8,7 @@ module Grape
   # should subclass this class in order to build an API.
   class API
     # Class methods that we want to call on the API rather than on the API object
-    NON_OVERRIDABLE = (Class.new.methods + %i[call call! configuration compile! inherited]).freeze
+    NON_OVERRIDABLE = %i[call call! configuration compile! inherited].freeze
 
     class Boolean
       def self.build(val)
@@ -51,6 +51,8 @@ def initial_setup(base_instance_parent)
       # Redefines all methods so that are forwarded to add_setup and be recorded
       def override_all_methods!
         (base_instance.methods - NON_OVERRIDABLE).each do |method_override|
+          next if Class.method_defined?(method_override)
+
           define_singleton_method(method_override) do |*args, &block|
             add_setup(method_override, *args, &block)
           end