make fallthrough accessors compatible with ruby 3

`method_missing` does not swallow kwargs on ruby 3 anymore

lib/mobility.rb

@@ -73,6 +73,9 @@ class Comment
 plugin enabled.
 
 =end
+
+def ruby2_keywords(*); end unless respond_to?(:ruby2_keywords, true)
+
 module Mobility
   # A generic exception used by Mobility.
   class Error < StandardError

lib/mobility/plugins/fallthrough_accessors.rb

@@ -55,6 +55,13 @@ def define_fallthrough_accessors(*names)
           end
         end
 
+        # Following is needed in order to not swallow `kwargs` on ruby >= 3.0.
+        # Otherwise `kwargs` are not passed by `super` to a possible other
+        # `method_missing` defined like this:
+        #
+        # def method_missing(name, *args, **kwargs, &block); end
+        ruby2_keywords :method_missing
+
         define_method :respond_to_missing? do |method_name, include_private = false|
           (method_name =~ method_name_regex) || super(method_name, include_private)
         end

spec/mobility/plugins/fallthrough_accessors_spec.rb

@@ -40,6 +40,22 @@ def method_missing(method_name, *args, &block)
       expect(instance.foo(**options)).to eq([options])
     end
 
+    it 'passes kwargs to super when method does not match' do
+      mod = Module.new do
+        def method_missing(method_name, *args, **kwargs, &block)
+          (method_name == :foo) ? [args, kwargs] : super
+        end
+      end
+
+      model_class = Class.new
+      model_class.include translations, mod
+
+      instance = model_class.new
+
+      kwargs = { some: 'params' }
+      expect(instance.foo(**kwargs)).to eq([[], kwargs])
+    end
+
     it 'does not pass on empty keyword options hash to super' do
       mod = Module.new do
         def method_missing(method_name, *args, &block)